This study was conducted to reveal primary school students' mastery of the concept of fraction and its seven sub-concepts. Sample of this study consisted of 160 fifth grade students purposively recruited from primary schools in the Municipality of Ujung Pandang and the District of Sinjai, South Sulawesi. Data were collected by means of a test on the mastery of concept of fraction consisting of 28 items. Results of the study indicated that in general the students had not yet mastered the concept of fraction sufficiently. Only in some of the seven sub- concepts the students' mastery was good but in other sub-concepts the students' mastery was poor to very poor