This study used the multiple methods of classroom observation, interview with teachers and students, and paper-and-pencil test to investigate the ability development of seventh-grade students in learning two-dimensional linear systems of equations and the corresponding error analysis. Hopefully, the results of this study can be as a reference for the middle school math teachers to plan the suitable teaching strategies when they teach two-dimensional linear systems of equations to their students. At the beginning, the researcher entered two seventh-grade classrooms of one middle school in Kaohsiung to make the preliminary observations and let students (also the teachers) to get used to the appearance of the researcher in the classroom during the period that one-variable linear equations were taught. Subsequently, the formal observations were carried out for 40 class periods that two-dimensional linear systems of equations were taught. All the observations made about how teachers taught and how students learned were recorded and content analyzed. Two paper-and-pencil tests were administered during the period of preliminary observations. And three paper-and pencil tests were given during the period of the formal observations. All the test results were collected and analyzed in numerous ways. Based on the literature survey and the interviews with six middle school math teachers, all relevant abilities of mastering two-dimensional linear systems of equations were classified into three categories: Character Symbols (10 sub-abilities), Operational Principals (five sub-abilities), and Other Abilities (16 sub-abilities). Based on the results of the content analyses of classroom observations and the error analyses of five paper-and-pencils tests for each sub-abilities of mastering the subject, it was observed that during the period of developing the abilities on solving two-dimensional linear systems of equations, most students showed some signs of obstacles and puzzles. Even by the end of the course on two-dimensional linear systems of equations, most students still did not master the subject well. Based on the results of this study, it is proposed that the length of teaching period needs to be increased and more efficient learning strategies need to be introduced to the students when two-dimensional linear systems of equations are taught.