The present study investigated effects of agreement type and morphosyntactic regularity on L1-Japanese learners' acquisition of explicit and implicit grammatical knowledge of the English plural morpheme. We adopted a timed grammaticality judgment task (TGJT) to measure implicit knowledge and an untimed grammaticality judgment task (GJT) for explicit knowledge in order to observe differences between learning difficulties in each type of knowledge. Twenty six participants completed the two tasks, and the scores were submitted to a three-way ANOVA (task types, agreement types, and morphosyntactic regularity). Only the main effect of morphological regularity was statistically significant in TGJT, while the interaction between agreement type and morphosyntactic regularity was observed in GJT. This result indicated that the two linguistic factors affect the acquisition of explicit and implicit knowledge in different ways.