This study aims to determine the lateral thinking process of students in terms of learning achievement in mathematics. The subjects of this study were six students of SMA ABAD with each level of high, medium and low mathematics learning achievement. Data were collected by means of mathematics learning achievement tests, lateral thinking tests and interviews. Data analysis used qualitative data analysis techniques, namely: data reduction, data presentation, and drawing conclusions. The results showed that the lateral thinking process of students with high mathematics learning achievement had been fulfilled based on lateral thinking indicators. In other words, the better the mathematics learning achievement, the better the students' lateral thinking ability. The lateral thinking process of students with moderate learning achievement in mathematics has been partially fulfilled based on indicators of lateral thinking. If the mathematics learning achievement is moderate than the lateral thinking ability is also moderate. The lateral thinking process of students with low mathematics learning achievement has not met the indicators of students' lateral thinking. The lower the mathematics learning achievement, the lower the students' lateral thinking ability.