This study was conducted in order to investigate the effectiveness of trait based assessment on teaching writing skill for EFL learners. Designed as pre-experimental study with one group pretest and posttest design, it examined 20 students of the second semester of English Department of "Hamzanwadi University" in the academic year 2016/2017 as the samples. Purposive sampling technique was used in determining the samples. Writing test and analytical scoring rubric were the instruments used to collect the data. Then the data were analyzed by using descriptive statistics and paired sample t-test to test the hypothesis. The result of descriptive statistics analysis revealed that trait based assessment "is" effective on teaching writing skill for EFL learners since the mean score of posttest 60 was higher than mean score of pretest 28.20. While for hypothesis testing by using paired sample t-test at significance (2-tailed) value level was 0.000, it was lower than 0.05. Therefore, it means that the hypothesis of this study was accepted. In other words, trait based assessment was significantly effective in improving students' writing skill.