The purpose of this research is to describe the source of errors found in learners’ writing products. In analyzing the data, the researchers used a qualitative research design. The design used to investigate the source of errors and the most frequency of errors found in learners’ writing products written by SMA students in Sintang. The data in this research was the students’ writing products. The data was taken from the teacher’s documents or taken directly from the students. The determination of subject and object of the research used purposive sampling. The finding shows that most error found in the learners’ writing products was intralingual errors. The most learners’ errors were classified as intralingual errors. It happened due to overgeneralization, ignorance of rule restriction, incomplete application of rules, and false concepts hypothesized. This research confirmed that most students made errors because of the students’ lack of competence in the target language.