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Justification ability can support other mathematical abilities and dispotions and can confirm student-s understanding. It is very important to able to know the student-s justification abilities. The purpose of this study was to determine mathematical justification ability of the first middle student, using qualitative methods from 21 student in one of junior high school in Bandung. Students are given triangle and quadrilateral question to do justification. The result showed that there are 3 level justification of junior high school student. The first level is students assume based on their own thinking, the second level is student explain with mathematical concept but are less precise and the third is student explain with the right and complete mathematical concepts. But when justification involving calculatios, generally student can do justification correctly only a few student make mistake because of miscalculation.