One of the most significant current discussions in L2 classroom research has been related to the necessity of vocabulary in language learning.Undoubtedly, EFL learners with high levels of lexical knowledge perform better in writing and oral tasks. Accordingly, the present study sought to investigate the efficacy of Task-induced Involvement in Incidental Lexical Development of Iranian Senior EFL Students. For this purpose, based on the scores obtained from an Oxford Placement "Test" ("OPT") administered to the population of senior EFL students at Khorasgan University, six samples, twenty five each, were selected and assigned to work with a list of English words by utilizing six different tasks. Each task was gauged by applying a different involvement load. Subsequently, a receptive and a productive vocabulary tests were administered as post-tests to specify the degree of learners acquisition of target words, through the role of incidental Task-induced involvement load. The results revealed that the group doing the task with the highest degree of involvement load obtained the best results on the vocabulary tests. Afterwards, the retention of unfamiliar words was claimed to be conditional upon the amount of involvement while processing these words.