The purpose of this study is to investigate the efficacy of interaction using Wiki in communication-oriented classes taught by native teachers on the incidental improvement of productive vocabulary knowledge. Eighty two university freshmen participated in the study and they were divided into two groups. Thirty eight students(Process Group) received process feedback from the teacher while forty four students(Result Group) received result feedback. They were given a short movie project working in a group of four or five people to practice interaction between peers and between students and the teacher. Data were collected from one receptive and three productive vocabulary knowledge test scores. The results showed that interaction using Wiki was conducive to the growth of productive vocabulary knowledge incidentally due to continuous opportunities to use vocabulary. The students of process feedback group made greater gains of productive vocabulary than those of result feedback group. Based on these results, pedagogical implications are discussed.