The objectives of this study were; to investigate the performance level of the fifth female graders, and to investigate the students' strategies used when computing mentally, and to examine the relationship between mental computation and basic computational skills. The sample of the study consists of (279) fifth female graders in three schools in South of Dhaherah District in Sultanate of Oman. For data collection, the researcher constructed two instruments, a Mental Computation Test and a Basic Computational Skills Test. After manipulating the validity and reliability of the two tests, the researcher administrated them in research sample. Researcher also conducted interviews with students having higher performance than the educationally accepted criterion (40%) in mental computation test. The study results reveale the following: 1- The average of students' performance on the mental computation test (24,14%) is lower than the educationally accepted criterion. 2- There are significant differences between the averages of students' performance in the computational operations (except the difference between the averages of the addition and subtraction, and also except the difference between the averages of the multiplication and division). 3-. The most strategies used by students is "the use of written algorithms mentally". Although the narrow used of mental strategies by students, these strategies indicates their high skills in mental computation. The most strategies used by students are; Work from the left to the right, Decomposition, Repeated addition and Distripution. 4- There is a significant relationship (r = 0.55) between the mental computation and the other basic computational skills The researcher recommendes to encourage students to use the mental computation as first computation method. She also emphasises the importance of continuous evaluation of students' performance. Researcher also recommendes conducting further studies on mental computation.