How to boost participative behavior of members is a critical problem for many open online platforms: although the number of registers can be huge, their actual participation is often limited.  In this study, we examine two widely used strategies for motivating registers’ participation : monetary incentive and social comparison.  Using a randomized field experiment conducted on a leading massive open online course (MOOC) platform, we find that monetary incentive boosts registers’ average participation score but social comparison decreases the score.  When the two are implemented together,  their interaction effect is contingent on the nature of social comparison.  The interaction effect is positive when social comparison is private, whereas negative when social comparison is public.  The findings suggest that the combination of private social comparison and monetary incentive is the most effective in motivating students’ participation in MOOCs, but social comparison itself is least effective.  The possible mechanisms responsible for the findings are discussed.