Little attention has been paid to the problems about portfolio keeping in English as a foreign language (EFL) writing, while the existing literature mostly focuses on the effects of portfolios on writing skills of learners, rather than those of teachers or pre-service teachers. This study aims to investigate the problems encountered and contributions of portfolios to the language skills of EFL pre-service teachers. The sample group consists of 39 pre-service teachers; a background questionnaire, interviews, a survey, and essays were used for data collection. The results indicate that portfolios significantly contribute to the writing skills, and that there exist potential problems. It was concluded that teachers and teacher trainers should use portfolio as a learning tool after solving the problems.