This research referred to Basic Competency Test from 2001 to 2006 to construct test and analyzed 207 ninth-graders¡¦ problem-solving strategies as well as errors in solving linear equations with two unknowns. Furthermore, the investigator referred to the contents of interview, to investigate the factors that cause students¡¦ mistakes. Results shows that the main strategy for solving equations is 'to add and subtract the elimination approach', while for solving application problems is 'organizing side by side'. The errors for solving equations are mistaking concepts including Equality Axiom, etc. The errors for solving application problems are mostly concerned about the translation and holistic mistake. Through analyzing data from interviews, the reasons for mistakes in solving equations are: mutual interference of experience; mixed up different operation rules; or, solving a problem with the wrong concept built by themselves. The reasons for mistakes in solving application problems are: insufficient language ability; the lack of the self-monitoring; and limitation in strategies for solving problems. Finally, based on the results of this research, the researcher gave suggestions in three aspects. Hopefully, this research can assist teachers to have more variety in teaching methods heading towards an aim to benefit in students¡¦ learning.