This research is administered according to the research objectives and questions that govern its parameters. It examines how portfolio assessment implemented in an English proficiency course promotes learner autonomy among Malaysian tertiary learners. A case study approach has been selected as it provides a suitable context to accomplish the aim of the study. To achieve the objective of the study, Sociocultural theory (SCT) and a reconceptualized construct of learner autonomy are adapted. This paper will discuss the use of portfolio assessment and its development of the psychological dimension of autonomy among English as Second language (ESL) tertiary students of different ethnicity. In this pilot study, data was collected from two students via background questionnaire form, face to face interview, and observation field notes. Participants were selected through purposeful, and homogeneous sampling. It is found that these participants experienced the motivational and affective sub dimensions of the psychological aspect of autonomy while completing their portfolio assessment. Findings from this study highlighted how portfolio assessment enables the participants to practice autonomous learning in its different psychological sub dimensions due to conditions such as self-confidence, portfolio assessment process, portfolio requirements, teacher factor, and the identification of strengths and weaknesses. It is hoped that the findings would benefit the ESL instructors and learners in their attempt to understand and consequently improve the teaching and learning of the writing skills by using portfolio assessment and provides insight into the practice of autonomous learning among learners of different cultural background.