Learning vocabulary is an essential part of language learning. The aim of the paper was to investigate the impact of different tasks on Iranian EFL students' vocabulary learning in Payam-e-Nour University in Bushehr, Iran. The total number of sixty male and female students participated in the study.Homogeneity of the subjects was ascertained by a General English Proficiency Test (Nelson Test). The test consisted of 50 multiple choice vocabulary, grammar and reading comprehension items. In this study, three kinds of tasks such as reading comprehension, reading comprehension with fill-in gaps and sentence writing used to collect data via pre-test and post-test. They were devolved by Richek' book [1] based on the target vocabulary learning. The data analysis revealed that the significant impact of task involvement on the incidental learning of vocabulary by EFL learners. {\textcopyright} IDOSI Publications, 2013.