Learning mathematics has a clear, ordered and complete structure or pattern of thinking. Fragmentation of the thinking structure occurs at a time which is a factor inhibiting students' thinking processes to construct the material concepts they get and in solving mathematical problems. This study aims to analyze the occurrence of fragmentation of thinking structures by looking at students' errors in solving problem-solving abilities on the material of linear inequalities of two variables. This type of research is qualitative research. A search was conducted on the occurrence of errors in the selected research subjects by carrying out tests with the think aloud method and task-based in-depth interviews in order to obtain data on students who experienced fragmentation of thinking structures on the material of two-variable linear inequalities. The results showed that there was fragmentation of the thinking structure in students' minds, especially in constructing concepts and reviewing the results of solving these problems. Each student seems to have fragmentation of different thinking structures according to the level of understanding that students have. Students are not able to use mathematics according to the actual concept of inequality and the occurrence of fragmentation of thinking structures that hinders students' thinking processes.