Danish toy maker Lego A/S is giving its colorful bricks their first makeover since 1963. But it will take 15 years and, if the company can help it, you won’t be able to tell the difference.

Executives are assembling a team of scientists to find a bio-based alternative to the petroleum-based plastic that has been used to make its bricks for decades because the popular building blocks are a significant contributor to the company’s carbon footprint.

It is expected to be a lengthy process, going to 2030, because none of today’s alternatives to plastics, such as those made from raw materials derived from plants, meet the company’s requirements for giving Lego bricks a consistent look and feel, said Tim Guy Brooks, the company’s senior director of environmental sustainability, in an interview.

“The ultimate prize would be not to notice one brick from another” when looking at old Legos made from Acrylonitrile Butadiene Styrene, or ABS, and new bricks that the company eventually hopes to produce, Mr. Brooks said.

And each of those pieces, whether basic blocks or the swiveling parts of characters in sets, must interlock with other pieces with unvarying precision.

The Danish toy maker is giving its colorful bricks their first makeover since 1963. But it will take 15 years to find a plant-based alternative to the petroleum-based plastic that it has been using for decades to make its bricks. Photo: Dan Kitwood/Getty Images

Making Legos is “incredibly precise. We mold it to about four-thousandths of a millimeter,” said Mr. Brooks. ABS is “very durable, holds color really well…it even has a particular sound.”

With the steep decline in oil prices, Lego seems to be running against the economic tide for a business centered on plastic. Attention to bio-based plastics grew significantly about 10 years ago when oil prices skyrocketed, pushing companies to search for alternatives to petroleum-based products, said Kent Furst, analyst for Freedonia Group, a market-research group. But for many, the urgency faded when prices started coming down.

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With a 15% increase in profit in 2014 to 7.03 billion Danish kroner ($1.05 billion) and a 24.6% operating profit margin, cost might be less of a motivator for Lego than growing consumer awareness of environmental issues.

Some 55% of global online consumers say they were willing to pay more for products and services from companies committed to positive social and environmental impact, according to a survey by Nielsen NV.

Consumer companies such as Lego have incentives beyond cost because they “cater to consumers who care a lot about producing carbon emissions and all the other nasty things associated with the development of oil and oil-based petrochemicals,” Mr. Furst said.

Coca-Cola Co., for example, has been studying the use of plant-based materials to make its plastic bottles. The company unveiled a bottle last month it said was made entirely from plant materials, including sugar cane and waste from sugar-cane manufacturing, though it could be years before most consumers see the new bottle on shelves.

Mr. Furst said Lego is better positioned to spearhead plastics research than most companies because plastic is at the core of its business and Lego is a vertically integrated company. “They make one product, and the one product they make is from one kind of plastic. It makes sense that they’re able to focus on coming up with the best bio-based material,” he said.

“ The ultimate prize would be not to notice one brick from another. ” — Tim Guy Brooks, Lego senior director of environmental sustainability

Lego, which used 77,000 metric tons of petroleum-based plastic granules to manufacture 60 billion bricks and other pieces for its sets in 2014, plans to recruit 100 researchers over the course of the project.

Mr. Brooks said they won’t rule out any possibilities in their search for alternatives, but Lego prefers that their new plastic be derived from waste materials, such as corn stalk or other agricultural waste “that doesn’t appear to serve any other purpose.”

“This is the right thing to do for Lego—fossil fuels are a finite resource and we know that,” he said.

Write to Loretta Chao at loretta.chao@wsj.com

Corrections & Amplifications:

Lego A/S used 77,000 metric tons of petroleum-based plastic granules to make its bricks last year. An earlier version of this article incorrectly stated it used 77,000 metric tons of petroleum. July 17, 2015