Taiwan’s new MMM legislative electoral system first implemented in 2008 was a sharp departure from the half-century long SNTV system. This paper examines effects of knowledge of the new electoral system on citizens’ decisions to vote or not to vote. Existing literature on voting either assumes that voters are fully aware of the electoral system and thus ignores the effect of knowledge or at best assumes it is an exogenous factor. This study distinguishes itself from other related works in three respects. First, we do not assume that voters are fully aware of the new two-ballot electoral system and make their voting choices accordingly. Instead, we design a set of four survey questions to measure the degree to which citizens understand the new electoral rule. Secondly, instead of summing up the number of correct answers to these survey questions (i.e. the traditional “raw score” approach) , we use a two-parameter item response model to estimate item loadings and then construct a continuous measure of latent knowledge. Thirdly, instead of assuming knowledge is exogenous, we build a two-equation simultaneous probit model to account for the effect of electoral system knowledge on voter turnout.This model is meticulously specified so that it allows for knowledge to be endogenous. We find that knowledge of electoral system is indeed endogenous and, in both SMD and PR ballot voting, higher knowledge of the new MMM system stimulates higher probability of voting after taking into account the endogeneity of knowledge.