This study investigated the effect of the portfolio assessment strategy to teach and assess writing performance of EFL third year students enrolled at the 3rd Secondary School, Skaka (Al-Jouf region). The purpose of the study was to determine whether a specific portfolio assessment model was effective in helping the students to improve their English writing performance in general and writing product skills and writing processes in particular. The targeted population consisted of 63 female students. The control group students (N=30) received traditional classroom instruction whereas the experimental group (N=33) received treatment (portfolio assessment strategy). Data were collected through English writing assessment test, analytic scoring rubric (a composition grading scale), self-reporting questionnaire on writing processes, portfolio holistic scoring rubric, and portfolio assessment model for EFL writing instruction. Test scores and self-reporting questionnaire scores were used as measures of students' English writing performance. Descriptive statistics, independent samples t-test, and Spearman rank order coefficient of correlation were used for data analysis. The results of the pre-administration of instruments indicated that the two groups were homogenous and that their English writing performance was poor and inadequate. However, the findings of the post-administration showed a remarkable improvement in English writing performance of the portfolio group students in general and in their writing product skills in particular as compared with the non-portfolio group. Likewise, findings indicated a statistically significant increase in the students' use of writing processes as a result of the portfolio assessment strategy. Moreover, a positive correlation between the students' (experimental group) scores in the portfolio and their English writing test scores was found. Using portfolio assessment, as a complementary to traditional tests, in teaching and assessing EFL writing was recommended and that assessment should be an integral part of the teaching process. The study concluded that the portfolio assessment model is found to be an effective instructional strategy as well as an evaluation tool and that it enhances the students' English writing performance by focusing efforts on writing products as well as writing processes. Based on the findings of the study, some recommendations are emerged: using portfolio in EFL writing instruction as a teaching and assessment strategy not to substitute for traditional tests; rather they complement each other. In addition, assessment process should be an integral part of everyday teaching practices students involve in. It is also recommended to offer training for EFL teachers in planning and implementing portfolios in writing classes in Saudi Arabia.