The present study was an attempt to investigate the impacts of different task complexity conditions including ± planning time and ± few elements on EFL learners’ use of transitional devices in writing tasks. To this end, 60 intermediate EFL learners from three language institutes in Iran were randomly selected and were assigned to four groups of 15 participants. Each of the groups participated in 9 treatment sessions of instruction. At the beginning of every session the researcher introduced some transitional devices with a brief explanation about their meanings and usage and after that, the participants of each group were provided with a writing task that required them to write a paragraph about a special topic using all the transitional devices that were presented to them in that session. The number of correct uses of transitional devices in each participant’s writing was calculated and reported in percentage and the average of every participant’s performance during all 9 sessions was regarded as that participant’s single score for the purpose of making comparisons between the four groups. The data was analyzed through Kruskal-Wallis test and the findings revealed statistically significant differences among the four groups in doing the tasks. At the end, a questionnaire with 20 Likert-type items was utilized to explore the participants’ perspectives of the role of task complexity in their writing. Consequently, the study revealed that using tasks with different levels of complexity can play a great role to improve the learners’ grammar in terms of transitional devices. Manuscript profile