By adopting a constructivist approach, a portfolio assessment system has been implemented to see the effects of the implementation on critical reading and learner autonomy. The study also attempts to explore students' feelings of the change they underwent as a result of the implementation. The study was conducted with 43 preparatory class students studying at Çukurova University English Language Teaching Department in the academic year of 2005-2006. The data collection and data analysis was done via adopting a naturalistic/interpretative approach. Data were obtained from a set of interviews, a focus group interview, written documents, and the autonomy and critical reading checklist. The findings of the study may have significant implications for the use of portfolio to enhance critical reading and foster learner autonomy in preparatory reading classes.