The purpose of this study is to compare the effects of computer assisted project-based instruction on learners' achievement in a science and technology course, in a computer course and in portfolio development. With this aim in mind, a quasi-experimental design was used and a sample of 70 seventh grade secondary school students from Org. Esref Bitlis Primary School in Istanbul was selected. The students were randomly assigned to one of the two groups: the control group and experimental group. Instruction to control group was provided by the traditional project-based learning approach, to the experimental group was provided computer assisted project-based learning approach. Both groups were given instruction on the topic of Our Living Conditions in the syllabus of 7th grade Science and Technology course. During the five-week instruction, experimental group was required to prepare the projects with using computer assisted project-based method, while control group used traditional project-based method. At the end of the study, the findings revealed that science and technology and portfolio assessment scores of the experimental group were significantly higher than that of the control group. This result indicates that the learning gains are higher when instruction is provided by computer assisted project-based instruction than by the traditional method.