The purposes of the study were to examine the effectiveness on the CAI in functional vocabulary learning for elementary school students with mental retardation. This study used the changing criterion design of the single subject experimental design. The learning experiment of the study included three CAI learning phases which included five functional vocabularies in each phase. If the selected vocabularies correction rate of the each phase reached 80%, the student could move to next phase. Two students with mental retardation were selected from the resource classroom in the east of Taiwan. Each of them took the pre-tests more than three times individually. Each student was taught by the researcher using CAI learning material in the length of forty minutes a day, four days a week. The post-tests were given after four weeks of the learning experiment. These extra tests were conducted three times for every two weeks in order to examine the learning rate of retention. Based on the results of the study finding, the main conclusions of the study are: 1.The CAI learning by using the U3 system (which is a CAI design tool for special needs) is more effectively for the mentally retarded students to learn functional vocabulary. 2.The selection of the functional vocabulary to teach the mentally retarded students in the ecological environment could facilitate the students' learning and generalization abilities. 3.The content of CAI materials should be based on the prior experience of the mentally retarded students as well as the concrete objectives from simple to difficult. 4.To avoid the boringness, the content of CAI design could add more social communicative language in order to increase the student interaction. 5.Traditional teaching and CAI could be used simultaneously in order to increase the learning efficiency. 6.The learning content that used the situational design could be useful based on the researcher's finding.