This research used mixed method with sequential explanatory strategy. This research aimed to (1) determine average achievement score in mathematical power ability test, (2) determine the mean score of mathematical disposition obtained from student's mathematics learning, (3) find out the influential existence of student mathematical disposition on mathematical power ability, (4) evaluate the achievement of indicators of dispositional mental function, and (5) describe the relation of mathematics dispositional mental function on students mathematical power in problem solving. Data source was 30 (thirty) eighth graders from a junior high school in Semarang, Indonesia. Methods of data collection were test, questionnaire, observation, and interview. Results of the quantitative study showed that mathematical disposition (X) had a significant (52.8%) influence on the ability of mathematical power (Y) by linear regression equation model Y = 34.022 + 0.442X. Results of the qualitative study indicated there was an internal consistency among three mental functions of mathematical disposition. Reciprocal relation also occurred between mathematical power and mathematical disposition.