The purpose of this study was to determine student's mathematical reasoning ability in social arithmetic material. This type of study is qualitative with descriptive method. The subjects used in this study were 37 students of 8 th grade junior hight school 13 Bekasi City then selected the subjects by purposive sampling namely one student who had high, medium, and low of mathematical ability. The research instrument used in this study namely one test in the form of an open-ended description on the indicator of mathematical reasoning ability which is to checking the validity of an argument in social arithmetic material. Data collection techniques in this research carried out were data that had been obtained analyzed with descriptive statistics and interviews were analyzed using the Miles and Huberman model. Based on the results of the analysis of the answer of each student's, namely students with high and medium of mathematical ability have been able to solve problem in social arithmetic material and meet one indicator of mathematical reasoning ability namely checking the validity of an argument, then student with low of mathematical ability have not been able to solve problem in the social arithmetic material and does not meet one indicator of mathematical reasoning ability namely checking the validity of an argument.