In an attempt to see how semantic aspects of the verb and subject affect the rate of erroneous L2 passivization of unaccusative verbs, the present study investigated 78 Korean college students` grammaticality judgment on English passive sentences. The results of the study showed that 1) the semantic type of unaccusative verbs is related to the degree of difficulty that L2 learners experience when judging grammaticality of passive forms of the unaccusative verbs; 2) the semantic type of the subject affects L2 learners` grammaticality judgments on passivized sentences; and 3) there was an interaction effect between the semantic type of the verb and the semantic type of the subject. It is argued that L2 overpassivization is a highly complicated phenomenon which cannot be explained without involving the effects of the semantic aspects of the verb and its subject, and the interaction of those two factors.