This study aimed to Designing aportfolio in mathematics to evaluate the performance of sixth grade students at the Institute of Intellectual Education in Jazan and assesses its relevance to the objectives of mathematics, and whether there are significant differences between the views of teachers and educational administrators on the appropriateness of the portfolio activities to meet the objectives of mathematics. A second aim is to look for the psychometric properties of the portfolio, and to test the relationship between degrees of student performance using files (portfolio) and their grades on the final math achievement test. The researcher used the descriptive method .The population of the study consisted of all mentally disabled sixth grade students in Jazan. The sample consisted of (55) students and (70) teachers and supervisors of special education. The portfolio and the final achievement test of mathematics were used as tools for the study. The results indicated that the quarterly activities of the portfolio were appropriate to the objectives of mathematics for sixth grade mentally disabled students. The results also concluded that the portfolio had proved to be a suitable tool for assessing the academic performance of students in mathematics since it obtained good reliability and validity. This result is further supported by the significant correlation coefficient between the grades of student performance using files (portfolio) and final achievement scores in math. Therefore the study recommended establishing special centers at the Ministry level to provide advice and assistance in using the portfolio and to offer intensive courses for teachers on how to use the portfolio in schools of special education in the Kingdom.