This study focuses on the use of pPortfolio as a tool in assessing students' English writing skills in a non-native English language context. The current assessment system in schools does not assess students' on-going development of English writing skills and leads to rote memorization and test anxiety. Portfolios provides the environment to learn without having the fear attached to tests or examination situation, as students get feedback at multiple stages. With that, portfolio assessment meets the purpose of assessment for students' learning. This study applied qualitative research paradigm while adopting an action research model in order to implement portfolio assessment in the school. Purpose sampling was done, with Grade Eight class being the senior most class of the school; and thus, they were more expressive and had better writing skills. Data for the portfolio was collected via students' samples of story writing, self and peer-evaluated work, reflections and teacher's feedback. Other method of extracting data was by interviews, observations and reflective writing. The students' work in their portfolio was analyzed at the end through an assessment rubric. The findings revealed that introducing the students to portfolio assessment increased their knowledge towards producing better work via peer/group learning, and they were able to monitor their own progress via reflections. The students also learnt about different assessment strategies (self and peer assessment) and dealt with the challenges they faced during the process. The implication that resulted from this research was that portfolio development process increased the workload of the students and completion of the portfolios took a lot of time. For that purpose, careful planning of different strategies and materials were modeled. However, the advantages for portfolios are envisaged to be more than the challenges; and thus, portfolios are recommended as a useful learning tool for assessing on-going development of students' English writing skills.