Researchers have been investigating various factors for differences in academic achievement among students besides the emphasis on cognitive intelligence. In fact, it has been found that academic achievement is not determined by cognitive intelligence alone. Among the factors that are thought to contribute to students’ success and academic achievement is thinking style. It stems from the belief that people have similar patterns in governing themselves just like society does. Studies have shown that people have different thinking styles – legislative, executive, and judicial style which do affect the way they prefer to do their tasks. Legislative style is considered as creative thinking, executive style is viewed as following instruction, and judicial style is represented as critical thinking. It has been found that students from Islamic private schools in southern Thailand, in general, do not perform well in public examinations. Hence, this study investigated the thinking styles of three categories of students and their relationship to academic achievement. Three groups of academic achievers among students from 3 selected Islamic private schools in Nakhonsrithammarat – in upper southern Thailand were identified as the sample. It comprised 158 grade ten students and the three groups were classified into low achievers (N=36), average achievers (N=102), and high achievers (N=20) based on the Ordinary National Educational Test (O-NET) score. The sample was obtained using stratified random sampling for low and average achievers while purposive sampling was used for high achievers. The descriptive statistics, multiple regression analysis, and Sternberg’s interpretation of the MSG scores were used in analyzing the data. The results reveal that the executive style was the predominant thinking style exhibited by all groups of academic achievers. Additionally, the judiciary style was the prevailing thinking style displayed by male students while the executive style was the most noticeable style presented by female students. Furthermore, only the legislative thinking style was found to be an indicator towards students’ levels of academic achievement. However, it was a weak one.