The aim of the present paper was to investigate the effects of processing instruction and traditional instruction on Iranian EFL learners' writing ability. Thirty participants who were non-randomly selected out of 63 Intermediate EFL learners, taking English courses in a language institute in Khoy-Iran, participated in this quasi-experimental study. All the participants were female with the age range of 18-24. The participants were randomly assigned into two experimental groups of processing instruction and traditional instruction. Two different instructional packages were used as treatment and two free writing tasks were employed as pre-test and post-test in this study. The results revealed that although both instructions had positive effects on Iranian EFL learners' writing ability, processing instruction group performed significantly better than traditional instruction. Some pedagogical implications of the findings were presented. The results might be helpful in syllabus design and teaching methodology.