Achievement test scores are used to diagnose strengths, weaknesses, and a basis for awarding prizes, scholarship, or degrees. They are also used in evaluating the influences of course of study, teachers, teaching methods, and other factors considered to be significant in educational practice. Still, sometimes there is a gap in the score of essay tests and the existing knowledge of examinees. In the present study, the relationship between writing skill and the academic achievement of Iranian EFL students was examined to find a logical connection between them. The results of four final exams as content scores were examined and scored again in term of writing ability in analytical scoring scheme according to IELTS criteria. Then the average of two sets of scores calculated by two raters was compared with content scores of the same tests. The results showed that correlation between content score of all students and their writing skills is meaningful at 0.01 level of significance. The results showed that there is a strong relationship between EFL students' degree of content score and their writing skill.