Effort-based decision making addresses how we make an action choice based on an integration of action and goal values. The nucleus accumbens (NAc) is implicated in allowing an animal to overcome effort constraints to obtain greater benefits, and it has been previously shown that cannabis derivatives may affect such processes. Therefore, in this study, we intend to evaluate the involvement of the cannabinoid system in the entire NAc on effort-based decision making. Rats were trained in a T-maze cost-benefit decision making the task in which they could choose either to climb a barrier to obtain a large reward in one arm or run into the other arm without a barrier to obtaining a small reward. Following training, the animals were bilaterally implanted with guide cannulae in the NAc. On test day, rats received cannabinoid agonist (Win 55,212-2; 2, 10 and 50 μM) and/or antagonist (AM251; 45 μM), afterward percentage of large reward choice and latency of reward attainment were investigated. Results revealed that the administration of cannabinoid agonist led to decrease of large reward choice percentage such that the animals preferred to receive a small reward with low effort instead of receiving a large reward with high effort. The administration of antagonist solely did not affect effort-based decision making, but did attenuate the Win 55,212-2-induced impairments in effort allocation. In agonist-treated animals, the latency of reward collection increased. Moreover, when the effort was equated on both arms, the animals returned to choosing large reward showing that obtained results were not caused by spatial memory impairment. Our finding suggested that activation of the cannabinoid system in the NAc impaired effort-based decision making and led to rats were less willing to invest the physical effort to gain large reward.