This study applied mixed methods with concurrent triangulation design that combines quantitative and qualitative research method in a balanced manner and convergence. Population in this study was as 188 students of 8th grade in a state junior high school in Semarang City, Indonesia. By using purposive sampling technique, 60 students were selected as the research subjects. The study aimed to observe the effect of mathematical disposition on students' mathematical power ability, and to describe how dispositional mental functions work when mathematical disposition is developed. Data was collected through written test (essay), questionnaires, observation and interview. Data of research result was then analyzed quantitatively (with analysis of variance) and qualitatively (analysis of interactive model). On the final stage, meta-analysis on the data was conducted. Based on the analysis of quantitative and qualitative data, it could be found that: (1) There was an effect of mathematical disposition (X) on mathematical power ability (Y) by regression equation model Y =37.106 + 0.567X which was linear on significance level = 0.05. (2) The teachers should optimize the dispositional mental functions of the students by selecting a learning model that could develop the cognitive, affective, and conative competencies together.