The medial prefrontal cortex (mPFC) including the anterior cingulate sulcus is implicated in both decision-making and social cognition, suggesting that this area may play a central role in decision-making based on social context. In the present study, neural activity was recorded from the monkey anterior cingulate cortex (ACC) while the monkeys chose one of two identical figures based on the choice previously made by a robot arm. Monkeys observed that the robot touched one of the two figures in the left or right side of a touch screen. Every time the robot chose the correct option, the same pair appeared on another touch screen for the monkey. Then, the monkey had to touch the figure in the same side to obtain reward. Neuronal responses were compared by one-way ANOVA among 17 intervals distributed in 4 phases: baseline before the trial, observation phase (robot arm choices and feedback signals), inter-phase interval (between observation and following execution phases), and execution phase (monkeys choices and associated outcomes). Of 264 neurons recorded, 164 (62.12%) responded in one or more intervals of the task. Of these, 16 responded during the observation-phase, 5 during the inter-phase interval, 98 during the execution-phase and 18 on both observation and execution phases. Furthermore, neuronal activity of 69 (26.14%) neurons during action observation was correlated with that during real action (execution). This type of neurons might correspond to mirror neurons. The results indicated that the ACC processes information about self and others actions and outcomes, which may support social-based decision-making.