Earlier research argues that educational programmes based on social cognitive theory are successful in improving students’ self-efficacy. Focusing on some formative assessment characteristics, this qualitative research intends to study in-depth how student teachers’ assessment experiences contribute to their self-efficacy. We interviewed 15 second year student teachers enrolled in a competence based teacher educational programme. Thematic content analysis results reveal that the assessment characteristics ‘authenticity’ and ‘feedback’ exert a positive influence on student teachers self-efficacy during all phases of the portfolio competence assessment. The results provide a fine-grained view of several types of self-efficacy information connected with these assessment phases.