Assessment is an integral part of instruction. As one form of assessment, self-assessment is relatively rarely used in writing class. This might be because its validity and reliability is still in question. This research aims at finding (1) whether students assess their writing tasks the same as their teacher does, (2) if they assess differently, why they do it differently. Twenty four students who were taking Writing IV course were involved in this study. The research method used was a mix of quantitative and qualitative descriptive. For one semester the students were taught to write an essay in English. Strategies on how to get an idea, to organize idea, to implement proper language use, to choose appropriate vocabulary, and to use mechanics (e.g. punctuation, spelling and capitalization) were intensively discussed. Along with the activities they were encouraged to implement the feedback received in their writing tasks. Finally, at the end of semester, an assessment was done. The writing task was not merely assessed by the teacher. The students were also given trust and responsibility to assess their own writing tasks. The results of the two assessments were compared. It was found that, in general, the results of assessment represented by scores given by the students did not much differ significantly from the scores given by the teacher. This implied that self- assessment can be as valid and reliable as teacher-assessment if students were properly and adequately trained by teacher. Key words: essay, score, self-assessment, teacher assessment