This study investigates how pre-school Thai children comprehend Thai short and long passives with adversative and non-adversative and action and psychological verbs. The results show that preschool children did not comprehend passives well, primarily because of the thematic role reversal in the subjects of passives and actives. Action verbs are acquired earlier and better than psychological verbs. Unlike previous studies of other languages, Thai children comprehend long passives better than short passives with most verb types.