This study targets to 1) describe students' mathematical reasoning skills in terms of guardian personality type; 2) knowing the causes of student mistakes in answering questions. This type of study is descriptive qualitative. The subjects in this study were students of class XI MIPA four Wanasaba 1 Senior High School in East Lombok in the 2020/2021 school year. The determination of the subject was done by purposive sampling and snowball. Data collection techniques used personality tests, tests of mathematical reasoning abilities, and interviews. The validation of each instrument was carried out by three experts in each field. The data analysis stage in this study was data reduction, data presentation, and conclusion. The results of this study indicate that the subject's mathematical reasoning ability only fulfills indicator 4, namely performing calculations based on certain rules or formulas. Subjects have not been able to meet other indicators. This is because the subject considers that the listed formula can be used in all types of triangles, does not know the rules of the cosine rule, the subject rarely practices and answers story questions, difficulties in linking the results obtained with questions, difficulty in choosing the formula to be used, and not careful in doing calculations.