This study aimed to describe the types of grammatical errors and identify the sources of errors in the seventh-grade students’ written descriptive texts. It used a mixed-methods research design, a combination of qualitative and quantitative methods within a single research project designed Creswell and Plano-Cark (2011) suggested. The data were collected using documents – students’ written descriptive texts – and semi-structured interviews. The study found 82 errors classified into – following the most-to-the-least frequent-error order – omission, misformation, addition, and misordering. Furthermore, this study identified four sources of errors: intralingual transfer, interlingual transfer, the context of learning, and communication strategies. This study argued that the factors considered to have affected their errors were the nature of the formative assessment and the national examination, emphasizing grammatical dimensions