There has been a significant volume of research studying the effect of individuals' motivation on reading comprehension. The findings of previous research have demonstrated both the direct contribution of motivation to comprehension and its indirect influence through mediating effects of reading amount and strategy use. The effect of the linkage between the two mediators, however, is somewhat underexplored in previous studies, meaning that we have not yet achieved a comprehensive understanding of the functioning of these mechanisms and especially the part reading amount and strategy use play in connecting reading motivation and comprehension. Also, it is still unclear whether the mechanisms governing this connection vary across different primary school grades. In light of these concerns, this study was conducted with 537 students from six primary schools in Hong Kong, including 266 Grade 4 and 271 Grade 6 students. All participants completed a reading comprehension task and a questionnaire survey. The results suggest that strategy use uniquely mediates the relationship between reading motivation and reading comprehension. No significant mediating effect through reading amount was found among either Grade 4 or Grade 6 children. However, for Grade 4 children alone, we discovered a two-stage mediating effect linking motivation and comprehension via (1) reading amount and (2) strategy use (i.e., reading motivation [right arrow] reading amount [right arrow] strategy use [right arrow] reading comprehension). The findings from this study enrich our theoretical understanding the effects of reading motivation on reading comprehension and also provide pedagogical implications for directions in future reading instruction.