On the basis of Schmidt's Noticing Hypothesis (1990) and theory of Craik&Lockhart's Depth of Processing (1972),an experiment has been conducted for comparing the effects of different reading tasks and reading purposes on L2 Incidental Vocabulary Acquisition (IVA) through ESP reading.In the present study,regarding a petroleum English document as the reading material,the author designed two reading purposes (discourse topic VS discourse detail) and four reading tasks (reading comprehension,writing composition,gap-filling and sentence-making).The results show that through reading ESP reading material,a large quantity of words could be incidentally learned by learners after finishing certain reading tasks with certain reading purposes.