Under the influence of Chomsky's idea of the innate language faculty, language aptitude is seen as a special ability which is separate from general cognitive ability in the field of SLA. Thus, some linguists(Krashen, 1985) argue that language acquisition is a natural process and the acquired language knowledge contributes to the actual use of a second language. Furthermore,language learning through teaching or error correction is a conscious process which only means the learner knows the language knowledge, but it does not mean the learner can use this learnt knowledge to express something. However, Cummins(1979) argues that there are two independent language abilities, i.e., basic interpersonal communication skills and academic language proficiency. BICS refers to the ability required for oral communication and it can be acquired naturally, while CALP is in relation to one's academic language ability which can be developed by formal instruction. Cummins' proposal has made a great contribution to the awareness of classroom instruction in developing learners' academic skills including academic writing in SLA. The principle goal of this paper is to consider the characteristics of academic essays and the writing strategies of them. The major claim of this paper is that an academic essay is written for the purpose of explaining something, comparing two different things, analyzing facts, or trying to convince readers to accept the author's perspectives. Therefore, it is significant to write an academic essay by employing the strategies of definition, serializing, comparison, classification, and critical analysis, etc.
