The mistake in problem-solving indicates the presence of the cognitive structure parts, which are problematic. One of the causes is thinking structures that need to be better organized and connected or experience cognitive mistakes. This study aims to describe students' thinking structure mistakes in solving PISA questions and the efforts to correct these mistakes through defragmenting thinking structures. This study uses a qualitative approach. This study uses a qualitative approach, including data reduction, presentation, and conclusion. Based on the results of the study, the mistakes of the students' thinking structures in problem-solving to finish the PISA questions were experienced by all subjects, including students misunderstanding the problem, mistakes in devising a finishing plan, carrying out a finishing plan, and rechecking process. These mistakes are influenced by several reasons, including the need for literacy skills in reading the meaning of the questions, not focusing on essential instructions on the questions, mastery of understanding the concept of prerequisite material, mistakes in operating, and the structured and analysis errors. The defragmenting process is carried out to correct students' thinking structure mistakes in problem-solving through intervention processes including scaffolding-review, scaffolding-restructuring, scaffolding-explaining, cognitive conflict, and disequilibration.