The aim of this study is to determine through teacher candidates' thoughts the effects of a portfolio assessment implementation on their metacognitive skills and attitudes towards a course on measurement and evaluation. Exploratory sequential mixed-methods design is employed within the study. The pretest/posttest control group design was used in the qualitative phase of the study and a semi-structured interview form in the quantitative phase. The study was conducted with the participation of 42 teacher candidates. Data has been gathered using the metacognitive skills scale, an attitude scale on the measurement and evaluation course, and a semi-structured interview form. While analyzing quantitative data, assumptions were detected using the SPSS 17.0 program (multi-normality, extreme values, emission, covariance matrices' homogeneity, linearity, absence of multi-link issues); descriptive statistics and MANCOVA analysis were also been made. The NVivo 8 program was used in the analyzing the quantitative data gathered from the semi-structured interview form. Portfolio assessments were determined to have positive effects on attendants' metacognitive skills and attitudes towards the course, and the implementation positively affected their attitudes.