Several studies suggest that most students are not in the same level of development (Slavin, 2008). From concrete operation level to formal operation level, students experience lateness in the transition phase. Consequently, students feel difficulty in solving mathematics problems. Method research is a qualitatively descriptive-explorative research aimed at comprehending the process of mathematics problem solving based on students' thinking level. Formal subject described in a structured manner so that there is no information that eliminated in the calculation process. While in transition subject, information which is constructed is only based on empirical knowledge. And on a concrete subject, thinking process can directly determine the solution of a problem. Students in formal thinking level are able to plan a problem solving by relating an information that is obtained to an information which is logically asked. Transitional thinking level are able to visualise the problems logically when the context of the problems are closely related to the experience they have. And concrete thinking level is only able to plan problem solving when the problem can be immediately and easily analysed.