The aim of this research was to examine the role of working memory in moderating the effects of automaticity on mental addition in 40 Chinese third-graders from the perspectives of task characteristics and individual characteristics. In Study 1, a 2 (working memory load [WML]) × 2 (automaticity), within-subjects design was utilized. There was significant interaction effect of WML and automaticity on students' response time in mental addition. The WML moderated the effect of automaticity on mental addition. In Study 2, we conducted an HLM analysis. Of all working-memory-capacity variables, only the phonological loop (with mathematics-based stimuli) was found to significantly moderate the effects of automaticity on students' mental addition.