This longitudinal study examined the contribution of morphological awareness to English as a Second Language (ESL) reading comprehension. Young Chinese-speaking ESL learners completed twice with a one-year interval the same set of tasks that measured derivational awareness, vocabulary breadth, fluency of reading derived words, and passage comprehension. Structural Equation Modeling analyses revealed that at both Time 1 and Time 2, morphological awareness had a significant direct effect on reading comprehension over and above vocabulary knowledge, and such an effect became stronger over time. A significant indirect effect of morphological awareness on reading comprehension was also observed at both times. However, how the indirect effect was achieved varied across times. Longitudinally, Time 1 morphological awareness failed to significantly predict Time 2 reading comprehension over and above Time 1 reading comprehension and vocabulary knowledge; however, an indirect effect through the latter two variables was significant. This study indicates an increasingly important role of morphology in ESL reading comprehension, and highlights a need of explicit teaching of morphology to facilitate ESL learners' reading development.