Whilst previous researchers commonly report on the effect of portfolio-based instruction on second language/English as a foreign language (L2/EFL) learners' language performance, very few studies examine its impact on their learning motivation. Drawing on expectancy-value theory, the current study examines how the implementation of a portfolio-based listening course may affect language learners' L2 motivation and skill development. This study adopted a mixed methods approach with Vietnamese EFL learners in higher education as its participants, who were administered a motivation questionnaire, listening comprehension tests, and semi-structured interviews. Both quantitative statistics and qualitative content analysis were applied for data analysis purposes. The results indicated that the portfolio-based program in this study had a significant positive impact on the participants' motivational orientations, especially their expectancy components (i.e., their self-efficacy and learning control beliefs) and their L2 listening accomplishment but not on the value aspects. Pedagogical implications are discussed.