In a Japanese agent-patient-action type sentence, an agent is marked by the nominative particle GA and a patient is marked by the accusative particle 0. The aim of the present study was to compare the cue strength of 0 with that of GA in sentence comprehension of agent-patient relations and to find their developmental process. Because 0 is semantically simpler than GA, it was hypothesized that (1) the cue strength of 0 was stronger than that of GA and (2) the acquisition period of the former particle was earlier than that of the latter in Japanese children. Eighty Japanese native speakers of 5, 6, 7, 9, 11, 13, 15 years old and adults were instructed to listen to simple sentences and to judge which noun was the agent in an act out method by using miniature animals and objects. Stimulus sentences consisted of 27 sentence types composed of word order×particle×noun animacy combinations. The results were compared with Ito and Tahara (1986). The hypotheses were both verified.