This study investigates SLA of English relative constructions by Japanese with the comparison between an instructed group and an uninstructed group. This research showed that instruction on English relative constructions had an effect on L2 learners’ sentence processing strategies that compensate for the difficulty in producing the target forms. Three kinds of sentence processing strategies were observed, i.e., Converting to Subject, Resumptive Pronoun, and Connecting strategies. The detailed analysis in the sentence processing strategies demonstrated a factor for Converting to Subject and Resumptive Pronoun is the Accessibility Hierarchy Hypothesis, and that of Connecting is Perceptual Difficulty Hypothesis.