Evaluation is essential to any learning and teaching process. Writing portfolio assessment has become increasingly used for evaluating learners' writing processes. Several scholars have proved that portfolio assessments have a positive impact on learners' learning process, especially on enhancing students' involvement and providing learners opportunities to learn from their own errors in writing. This study determined students' attitudes toward the use of writing portfolio assessment and examined the effects of writing portfolio in a module course. Students' opinions of portfolio assessment in the School of Foreign Languages of a south-eastern state university in Turkey were also studied to determine whether the portfolio assessment model was successful in helping learners to improve their writing abilities. The results revealed that most students generally possessed positive attitudes toward the use of portfolio. They claimed that portfolio assessments were very useful in assisting them in developing their writing skills, as well as positively affected their writing performances in quizzes and exams. In addition, a positive correlation was also identified among the scores of the portfolio, quizzes, and exams. This suggested that students with high scores on portfolio tended to achieve higher or similar scores on their writing quizzes and exams, and vice versa. This also indicated that performances on writing portfolio assessments may be predictive of students' writing performance on writing exams.