Triangle is one of the fundamental concepts in geometry which has to be studied in secondary schools. Students have many difficulties to implement maths' reasoning and to understand concepts of the triangle. If students' difficulties have to be addressed and facilitated, then the students understand the concept of triangles. One is called a mathematics justification. It is an important skill for students to develop mathematical reasoning for learning and understanding mathematical concepts. This study used a descriptive-qualitative approach aimed for assessing students' justification strategies in understanding triangle concepts. This study was conducted in the MTs' Hidayatul Hasan, Lumajang. Out of class, which was out of five classes of grade seven, were selected purposively as the sample of this. All students' were given both mathematical ability and justification tests. Six volunteer students, every two students with low, medium and high maths' abilities, respectively, were selected as respondents. All respondents were individually interviewed based on the answer to the justification test. Our results agreed to some findings that the geometry's concepts are only focused on computational skills, lack justifications in the understanding of what concepts and without explaining why the used strategies are appropriate or correct.