The aim of this research was to examine the effect of task complexity instruction on EFL pre-intermediate learner’s incidental learning of grammatical collocations through reading. To do so, the test of general English proficiency, OPT, was administered to 140 participants to homogenize subjects. Based on the mean score (X= 34.5) and standard deviation of students’ scores (SD= 2.8), 90 subjects were selected, those scoring between half a standard deviation above and half a standard deviation below. They were assigned in three classes with 30 students in each. Each of these classes was randomly assigned to one of the three tasks (fill in the blank, sentence writing, and translation sentences). Then the pretest, based on fifty fill in the blank questions was administered. After ten treatment sessions, the post-test which was the same as pretest was given to the participants to measure their knowledge of grammatical collocations in the tasks. Paired samples t-test, one-way ANOVA, and post-hoc tests were used to calculate for the productive and receptive knowledge of the students. The findings showed that there is significant main effect for all three groups. The result of this experiment is discussed in light of the involvement load hypothesis.