The main purpose of this research was to study Sidaamu-afoo interference in learning writing in English. To be specific, the study was intended to identify the aspects of the language interference and to describe the frequency of the interference of the aspects. It is a descriptive type of research. Three secondary schools found in Arbegona Woreda were purposively selected. From each of these schools, one section of grade nine was randomly selected to give a total of 176 study participants. Data were collected by a writing test and analyzed by using contrastive analysis and error analysis approaches; moreover, the data were put in tables and quantified and changed into percentile. The findings indicated that aspects such as L1 (Sidaamu-afoo) induced spelling errors, structural interference, lexical interference, wrong word use, and omission errors were committed by the students’ English language writing. The kind of transfer that dominated was spelling, structure, and omission errors. Accordingly, recommendations have been made.