Recently, the use of micro-computers has entered the field of education, including L2 English education. However, courseware should be carefully developed for effective learning, and for that purpose, many factors such as appropriate learning time and quantity of instructional material for each lesson on CAI should be investigated. We developed CAI courseware for vocabulary building and reading comprehension, consisting of 2 lessons. Each lesson of CAI courseware consisted of a vocabulary drill, and instructional material to be read three times, followed by a test of reading comprehension. In the reading section, students first read the material while listening to a tape recorded by a native speaker. The second time, they read it using a function for accessing each word's meaning in the context, dictionary function, and the third time they read it at their own pace. When students finished their learning on CAI, they filled out a questionnaire. Seventy-four subjects studied on this courseware, in the fall of 1986, and their learning time was recorded by the program on the microcomputer. All subjects were divided into three groups based on their reading grades in their college English course. The purposes of this study were as follows: 1) to clarify the relationship between a student's individual characteristics and his/her learning time for mastery learning of vocabulary, 2) to decide the appropriate quantity of instructional material for each lesson, from the point of learning time, and 3) to discuss the adequate level of dictionary entries in order to assist college studens in reading the instructional material. The scores of pre-tests, post-tests and retention tests of 2 lessons were calculated. With the scores of the pre-tests, analyses of variance were conducted, showing significant differences among three groups. However, the average scores of post-tests and retention tests of all groups were higher than nine out of ten points except one case, so it can be said that mastery learning of vocabulary was achieved on this CAI program. Secondly, analyses of variance were conducted of students' learning time of reading the instructional material three times. Signficant differences were seen only in the second time, lower-graded students spent about 1.3 times more time than higher-graded students. Based on this result, about 15 to 20 words of the reading material were thought to be appropriate for each lesson. Thirdly, dictionary entries were examined form frequencies of consulting each dictionary entry and the level of difficulty of each word or idiom. Though dictionary entries prepare words/idioms required as high school level or higher, all entries except one were used. All the entries used by more than 50 percent of all students turned out to belong to college level or higher. From these results, our standard of selecting the dictionary entries were thought to be suitable for college students. Moreover, the results of the questionnaire generally reflected students' positive attitude to the learning with CAI. However, lower-graded students expressed a significantly favorable impression of CAI learning.