Student's weakness in solving math problems raises the question of how students solve math problems by expressing ideas in solving problems. An explorative study was conducted to investigate how the ability of junior high students in solving mathematical problems in understanding and solving the problem by revealing students' functional thinking ability. Subjects were eighth graders of junior high school students consisting of 19 male students who then taken 3 male students as research subjects who have different abilities. Functional thinking has four stages, namely; Recognize rules, generate verbal rules, generate symbolic rules, and apply rules. The results showed that, the activity of students with high mathematics ability at that stage in sequence; can choose the rules correctly, mention the appropriate rules, write the rules symbolically, apply the rules correctly and prove it. In students with middle mathematical ability; can hoose the right rules, mention the appropriate rules, generate the rules symbolically, apply the rules correctly. Students with low mathematical ability: read the choice of rules repeatedly to choose the correct rules. Mention the appropriate rules, write the rules symbolically, the students misapply the rules, but when interviewed apply them correctly.