This study investigated the recognition of substantives in substantive-postposition clauses including case postpositions according to the liaison rule in children from three to five years of age learning vocabularies. Research subjects were 45 children aged three to five, 15 children from each age. In a task of the recognition of substantives, they listened to 36 sentences and, questions were asked. The children separated postpositions and responded with substantives. The older children performed the task of the recognition of substantives better, and all three age groups showed higher performances in the non-liaison condition than in the liaison condition and showed high performances with subjective postpositions. In general children actively learning vocabularies, if a vocabulary they did not know was combined with a postposition they knew, older children separated and recognized subjective postpositions, more easily. However, since they showed more excellent performances in a non-liaison condition at all ages, it would be hard to separate substantives in all postposition conditions if the liaison rule was applied.