The aim of this study is to determine the effect of using portfolios in fifth grade mathematics on students' cognitive strategy use and views about using portfolio. In the research, which was carried out in two separate groups, a pre-test post-test control group experimental design was used. As for data collection, “cognitive strategy use scale” and “the process of developing portfolio assessment form”, originally developed by Pintrich and De Groot (1990) and adapted to Turkish by Üredi (2005) were used (2005). The process of developing portfolio assessment form was developed by researchers. Covariance and descriptive analysis techniques were used for the analysis of data. Research findings indicate that the use of portfolio assessment has no effect on students' use of cognitive strategy. Students enjoyed producing their own products; nevertheless filling out forms caused boredom.