A man is dead and another is fighting for his life after the pair were struck by a car while cycling in Sydney's north-west.

The two men were travelling west on Old Kurrajong Road in Richmond shortly before 5am when they were struck.

One man was declared dead at the scene after emergency services arrived. The second man was treated by NSW Ambulance paramedics before being transported by helicopter to Westmead Hospital in a critical condition.

A male driver was taken to hospital for mandatory blood and urine testing.

Officers from Hawkesbury police area command established a crime scene and the crash investigation unit have commenced inquiries.

Kurrajong Road is closed in both directions and road diversions are in place.