This study aimed at clarifying the impact of using a strategy based on problem solving in developing the number-sense skills of grade five students in basic education in light of their achievement in mathematics. To verify that, a teacher's guide was prepared for the fractions unit according to the strategy based on problem solving. The researcher has also prepared a test for number sense skills and its validity and reliability were measured on a survey sample. The sample of the study consisted of (60) grade five students from a basic education school in Al Batinah North Governorate. They were divided into two groups: an experimental group who studied the unit of fractions using the strategy based on problem solving and a controlling group who studied the same unit using the usual teaching method. The study results showed that: • There is a statistically significant difference at the level of significance (a < 0.05) between the mean score of the experimental group and the mean score of the controlling group in the post number sense skills test for the favor of the experimental group. There is a statistically significant difference at the level of significance (a < 0.05) in gaining the number sense skills due to the interaction between the teaching strategy and the level of mathematical achievement among grade five students in basic education. In light of these results, the study recommended that it is important to encourage teachers to use the strategy based on problem solving in developing the number-sense skills with students. It also recommended to adopt the concept of number-sense and its skills as teaching goals in basic education and organize the content in light of the number sense skills.