The purpose of this research was to investigate the effect of using diagram- drawing strategy on solving mathematical word problems of Grade 10 students in Iran.. There were 40 students that participated in this study. The data collected via one test consisting of six problems. For this test, three types of diagrams namely; network, hierarchy and matrix, were applied. The test was administered twice; first time without diagrams and second time after one week, adding diagrams to that test. the effect of three types of diagrams on solving word problems, was investigated. The results showed that, the only one that made a significant effect on students’ improvement of solving problems, was matrix diagrams. The research is suggested that the appropriateness of diagrams in relation to the structure of problem is crucial. This implies that only adding diagrams to word problems, do not guaranty the students’ improvement in word problem solving.