Recent advances in computer technology have led to the creation and development of computer-assisted instruction (CAI), which are rapidly assuming a prominent role in many fields of education. The development process includes analyzing lesson and users, designing computing lesson and assessment techniques, constructing computing lesson and assessment tools, and evaluating the lesson through users feedback. In this work, we aim to (i) Develop computer-assisted instruction of immune system organs and immune system diseases, and (ii) Examine efficiency of the developed CAI. A total of 142 secondary school students was involved in the study. Pretest-Posttest and Satisfaction questionnaire for the students towards the CAI were used as research instruments. The quantitative data was analyzed by t-test, and the qualitative data was analyzed by content analysis. The paired-sample t-test indicated a significant enhancement of students’ understanding after the using of developed instruction at .05 significant level. Moreover, the students were very satisfied with the developed CAI.