This study aimed to investigate the effectiveness of mathematical modeling in improving grade five students' achievement and developing their problem solving skills. The study used a quasi- experimental approach and randomly selected two groups of (65) grade five students from Hafsabint Omar' school. One group was randomly assigned as experimental and the other as a control group. Data of the study was collected by the means of two tests: achievement test and problem solving skills' test. The reliability of the achievement test is (0.92), whereas problem solving skills' test is (0.88). Both tests were developed by the researcher. The study found that: - There is no statistically significancant difference at the level of (o = 0.05) between the mean score of the experimental group and the mean score of the control group in the post achievement test. – There is a statistical significancant difference at the level of ( = 0.05) between the mean score of the experimental group and the mean score of the control group in -understanding the problem as a major skill and it's sub-skils (determining the given& determining the needs), and in validating the solution skill as a sub-skill of looking backward skill, and no statistical significant differences found in the remaining skills and sub skills including devising a plan by carrying out the plan obtaining a logical solution as well as the overall skills. - There is a statistical significancant difference at the level of (oC = 0.05) between the pre and posttest of the problem solving skill test in whole test for both groups. In the light of the above results, the study ended with some recommendations such as: include the mathematical modeling in all math's curricula for all grades in the way that suits the different ages. This is might be an excellent way to familiarize them with this method from early stages. Add to that, conducting training workshops for supervisors and teachers to make them aware of the positives of the method and the suitable ways to apply it in the classrooms.