Task complexity has recently attracted great attention in second language (L2) studies. However, its potential impacts on learning transitional devices have not been considered. The current study was an attempt to analyze the impacts of manipulating task complexity conditions on EFL learners’ grammatical enhancement in terms of learning transitional devices through doing writing tasks. For this purpose, 75 intermediate EFL learners learning English in three English language institutes in Iran were randomly selected. They were assigned to four experimental groups and one control group (each with 15 participants). Each of the experimental groups was presented with a pretest, writing tasks, an immediate posttest and a delayed posttest. The participants took part in 9 sessions and in each session some transitional devices were introduced to the experimental groups with which they were supposed to write a paragraph based on a special topic using all those transitional devices. The different experimental groups received writing tasks with different complexity levels which were determined through the manipulation of factors including ± few elements and ± planning time. The participants in the control group just participated in a regular English class for 9 sessions without doing such tasks. The performances of all groups were analyzed, and the findings revealed statistically significant differences among the five groups in both the immediate posttest and the delayed posttest, after controlling for the effect of the pretest. The findings of the current study have practical implications for curriculum development and EFL writing instruction.