There are still many students, especially elementary school students, who make mistakes in arithmetic operations of integers. Of course, this must be helped immediately so that students do not experience further difficulties in learning mathematics. To help students correct these mistakes, it is important for teachers to know, not only the causes, but also how students reason during the arithmetic operations. This exploratory research seeks to trace the reasoning of students who have made mistakes in the arithmetic operation. Five students from grade 4 elementary school who made mistakes in an arithmetic operation test were interviewed to explore the causes of mistakes and their reasoning process. It was found that generally the cause of the mistakes was the limited knowledge of the prerequisites for performing arithmetic operations. Nonetheless, some traces of the students' reasoning showed noteworthy creative thinking. Teachers can use this reasoning trail to improve students' reasoning processes in arithmetic operations.