The current study examines the mental computation performance owned by students at fifth grade. This study was carried out with 118 fifth graders (11-12-year-olds) studying at 3 randomly selected primary schools that served low and middle socio-economic areas in a city of Turkey. "Mental Computation Test (MCT)" has been used to reach how participants mentally compute. In analysing data, participants' scores on the test were calculated by using a statistical package and which mental computation level they were in was determined. Sample responses of the some students regarding some questions in the test were presented directly and discussed. Evidence was found that the total percentage of the students at quite low and low level is 7.6% and at high and quite high level is calculated to be 78.8%. It was also revealed that the mental computation performances of the students in addition and subtraction are better than multiplication and division. On the other hand, it was found out that the performances of the students in all operations decrease as the number of the digits increases.