The objectives of this research were to construct and verify a mathematics diagnostic assessment on addition and subtraction basis for grade 4 students in Buriram Primary Educational Service Area Office 3. The research samples consisted of 440 Prathomsuksa 4 students selected by adapting the multi-stage random sampling method. The research instruments employed in this research comprised of two types of examinations constructed according to strand 1: number and algebra in Mathematics 1.1. The tests were to examine test and diagnostic test. The examining test was a subjective test, consisted of 80 items which divided into addition part for 40 items and subtraction part for 40 items. The diagnostic test was a multiple-choice with 4 choices tests, consisted of 48 items which divided into addition part for 24 items and subtraction part for 24 items. The diagnostic test. The results showed that the content validity, determined by the expert, had the IOC score at 0.60-1.00. The addition part in the 1st test had a difficulty score of 0.21-0.79, the discrimination score at 0.23-0.89, and the reliability score at 0.89. The subtraction part in the 2nd test had the difficulty score at 0.31-0.72, the discrimination score at 0.32-0.72 and the reliability scores at 0.32-0.81.