The purpose of this study was to explore the effectiveness of mental math strategies on developing number sense skills of third grade students with math learning disabilities. A related purpose was to explore how these strategies affects students' academic achievement in mathematics. The study population consisted of 85 male and female third grade students with math learning disabilities at Al-Buraimi Governorate, Sultanate of Oman. After data collection, the main sample was 20 male and female students who were divided into two equal groups: experimental and control. The administration of the program lasted for two months and consisted of 24 sessions. Each session took about 40 minutes. For data collection purposes related to diagnosis and hypotheses testing, the study used the Raven Colored Progressive Matrices Test (Kazim et al., 2008), a diagnostic test in mathematics, which was prepared by the Ministry of Education at the Sultanate of Oman, and number sense skills test prepared by the researcher, and a training program using mental math strategies prepared by the researcher. The results of the study indicated that there were statistically significant differences between the mean ranks of students' scores in the experimental group in pre- and post-testing for the number sense skills test and the diagnostic test in favor of the post-testing. Also, there were statistically significant differences between the mean ranks of students' scores in the experimental and control groups in the post-testing for the number sense skills test and the diagnostic test in favor of the experimental group. Keywords: Number sense, mental math, academic achievement.