The main purpose of this research is to discuss the interactive processes and the affective outcomes of the first-grade senior high students when they cooperate in mathematical problem solving. The subjects are four first-grade senior high students in Kaohsiung. Upon their wills, they are divided into two homogenous groups. This research adopts case study method and analyzes the solving protocol by thinking aloud, interviews and questionnaires. The main findings and results are as follows: 1. The peer-interactive graph is an efficient methodological tool to analyze the processes of cooperative problem solving between students. 2. By using this tool, we can draw a conclusion that the students in this case have a stable problem solving model. 3. When students cooperate to solve problems, the emotional support between members will strengthen their endurance on problem solving and their desire to challenge difficulties. Moreover, the affective outcomes, such as withdrawal and defense, caused by bad performance will influence follow-up problem solving behaviors.