Regarding the fundamental role of speaking in language skills, this study intended to investigate the effects of speaking portfolio as an alternative form of assessment for assessing Iranian EFL learners' speaking ability at the intermediate and advanced proficiency levels and also its impact on their attitudes. Accordingly, from the population of 72 students studying at Kowsar Language Institute in Esfahan, a sample of 64 male and female intermediate and advanced students were randomly selected based on their scores on an OPT test and they were assigned to 4 groups: intermediate and advanced experimental groups and intermediate and advanced control groups. In order to collect the data, a pretest and a posttest as well as a questionnaire were employed. To analyze the data, an ANOVA and a series of Chi-square were run in the study and the findings indicated that the experimental groups using speaking portfolios performed better than the control groups in terms of speaking ability. Moreover, the result shed light on the advantages of speaking portfolios such as self-assessment, peer-feedback, and improvement of speaking skill. This study provides instructors, administrators, and test developers with alternative ways to improve and assess speaking skill through speaking portfolios.