The purposes of this study were: (1) to analyze the trilateral relationship of self concept, life adjustment, and academic achievement of the mountainous junior high school students; (2) to research if the students' self concept, life adjustment, and academic achievement have difference by the influence of ethnic group and sex; (3) to investigate whether the factors of self concept and life adjustment could predict and interpret the students' academic achievement. The sample were taken from 338 ninth grade students which included 135 aboriginal and 203 non-aboriginal students of four junior high schools in Nantou mountainous region. Tennessee Self Concept Scale, California Test of Personality, and Junior High School Students Academic Achievement Test were revised or constructed, and administered, to the subjects. The obtained data were statistically treated by two-way analysis of variance and stepwise multiple regression analysis. The main findings of the study were as follows: 1. Self concept was positively related to life adjustment and academic achievement; life adjustment was positively related to academic achievement, too. 2. Self concept did not show the differences between ethnic groups, but there was a distinction between the male and female; that is the girls' moral-ethical self were scaled higher than the boys'. 3. Life adjustment has no difference by the influence of ethnic groups, but the girls' social adjustment were superior to the boys'. 4. In Chinese, society, physical science and mathematics, non-aboriginal students performed significantly better than the aboriginal students did, but there was no difference in English learning. Then, the girls' achievement in Chinese and English was superior to the boys'. 5. Moral-ethical self, social self, and social adjustment can account for and predict the students' academic achievement