Abstract
This study examined the effect of explicit and implicit instruction on second language learners' implicit knowledge of English past simple tense. Fifty-nine intermediate Persian learners of English took part in this study receiving either explicit or implicit instruction about the grammatical feature at classroom setting. Their ability to use the structure was measured at three times through a timed grammaticality judgment task and an elicited oral imitation task. Data was analyzed using ANOVA. Results showed explicit and implicit instruction promoted the use of the target structures in both timed grammaticality judgment task and elicited oral imitation task equally effectively. Findings of this study do not support superiority of explicit instruction over implicit instruction in promoting second language implicit knowledge.