This paper aims to explain how students' understanding of negative numbers and how the concept applied students in solving problems. Based on preliminary studies that have been done by researchers and from many studies, negative numbers became an obstacle for the students themselves because of its abstract concept. The study was conducted on 64 seventh grade students who were divided into two classes in different junior secondary schools. These students have learned about the negative integers in the main lesson of integers. Data taken from the video recording of learning and the work of the student's task in answering the problem solving. All data are analyzed descriptive qualitatively and categorized based on the characteristics of the answers and compare them with related theories. Some respondents who had the characteristic answers are further analyzed to trace their concept construction in solving the problem. The result of the study showed that there were students who had good mathematical skills in problem-solving but they had constrained in performing arithmetic operation involving negative numbers.