The purpose of this study is to determine the reasoning ability of sixth-grade students in solving basic geometry problems on a flat plane. The subjects of this study were 6 students from 24 students of the Integrated Islamic Elementary School "Al-Mawaddah", representing the leading public and private elementary schools in the city of Semarang. For research on three intellectual abilities, namely intelligent, moderately intelligent, and less intelligent, two students were obtained for each on the recommendation of the homeroom teacher. The research method used is a mixed method, which is a type of research in which a researcher combines elements of a qualitative and quantitative research approach. The data collection techniques were observation, written test, and interview test. The results showed that the research value exceeded the completeness value (= 70). The validity of the data was carried out by triangulation of different times, and valid data were analyzed to draw conclusions. The following is a profile of students' basic geometric reasoning abilities in solving problems as a form of mathematical ability. The results showed that subjects with high, medium, and low abilities met the indicators of ability and basic geometric reasoning skills, including visual, verbal, drawing, logic, and applied skills.