The present paper addresses Korean EFL learners’ acquisition of psych-verb and unaccusative constructions from a processing point of view. Korean learners’ interlanguage psych-verb constructions and unaccusative constructions are marked by underpassivization and overpassivization, respectively. While both kinds of errors are observed in Korean learners’ data, the processing account of language acquisition predicts that learners will commit underpassivization errors more frequently than overpassivization errors because passivization requires more processing. In order to see if the acquisition of psych-verb and unaccusative constructions is affected by the processing complexity of passivization, the present study compared learners’ performance on psych-verb and unaccusative constructions. Ninety six university students performed a timed grammaticality judgment task on the two types of constructions. The comparison of the learner performances between psych-verb and unaccusative constructions revealed that the learners were more accurate with psych-verb constructions than with unaccusative constructions. The learners’ accuracy with psych-verb constructions increased faster than unaccusative constructions, with the increase of the overall proficiency. The findings suggest that while processibility might be a major issue for low-level learners, there are other factors such as L1 transfer that exert a pervasive influence on the acquisition of psych-verb and unaccusative constructions.