The purpose of this study was threefold: 1) to investigate the distribution of 6 most common types of grammatical collocation errors in Iranian English-major students’ performance; 2) to see which types are more persistent in their performance; and 3) to find out the improvement of the participants’ knowledge of grammatical collocations by their language proficiency. To this end, 276 Iranian EFL learners were selected from among all those studying English as their major at Sheikhbahaee University, the University of Isfahan, and the Islamic Azad University of Khorasgan. They were 44 freshmen, 91 sophomores, 73 juniors, and 68 seniors. The participants’ levels of proficiency were assessed by a placement test adapted from Pearson Longman ELT, based on which they were placed in three groups of pre-intermediate, intermediate and upper-intermediate levels of proficiency. They were then given a 35- item test of collocations consisting of noun + preposition, preposition + noun, adjective + preposition, verb + preposition, verb + infinitive, and verb + gerund collocations. Their performance on the Test of Collocations was meticulously analyzed and the frequency of errors in each type of grammatical collocations was computed. The results showed that as they advanced to higher levels of proficiency, their performance on English collocations significantly improved. The frequency of the errors in the performance of each group indicated that noun + preposition, preposition + noun and verb + gerund collocation errors were the most frequent ones for Iranian EFL learners, respectively. On the other hand, for the pre-intermediate learners, collocation errors on verb + infinitive had the smallest frequency; while for the intermediate and upper-intermediate learners, collocation errors on verb + particle/preposition were the least frequent. A comparison of the order of the frequency of grammatical collocation errors from the most to the least frequent ones in the performance of participants showed that there was no one-to-one correspondence between the ranks of types of errors across the levels of proficiency.