The purpose of this research is to develop a multidimensional computerized adaptive test for diagnosing the cognitive process of grade 7 students in learning algebra by applying multidimensional item response theory. The research is divided into 4 steps: 1) the development of item bank of algebra, 2) the development of the multidimensional computerized adaptive testing program and a handbook for using the program, 3) the trial of the multidimensional computerized adaptive testing program and the handbook, and 4) the output evaluation of the developed process of the multidimensional computerized adaptive testing. The research and development output are as follows: The multidimensional computerized adaptive testing consists of the item bank. The item bank has criterion quality and it is divided into 2 parts: (1) item bank of order and graph, consisting of 59 items, (2) item bank of linear equation with one variable made up of 104 items. The multidimensional computerized adaptive testing was processed on Windows XP and Windows 7 and it could diagnose the cognitive process of grade 7 students learning algebra; it includes their ability to remember factual knowledge, to understand conceptual knowledge, to apply procedural knowledge, to analyze conceptual knowledge, and overall aid the classification of examinees into pass or fail categories. The trial of the multidimensional computerized adaptive testing program by the teachers and students indicated that they had satisfaction in processing the program. The evaluation of the multidimensional computerized adaptive testing by users indicated that it had a considerable efficiency in terms of utility, feasibility, propriety and accuracy. (Contains 3 tables and 15 figures.)