The purpose of the study is to identify linguistic complexities and register appropriateness of Korean EFL writers by comparing their argumentative essay writings with those of an NNS group and an NS group, based on the assumption that academic writing of higher proficiency writers contains linguistically more-complex words and sentences and stylistically less-conversational and more-academic features. The lexical density and syntactic complexity are measured for linguistic complexity, and conversational and academic features for register appropriateness. The analyzed results revealed that the Korean writers were so strongly influenced by the structural features of the essay topics they chose that they overused attributive adjectives, nominalization, and passive voice. They also appeared to lack the ability to employ varied and sophisticated words and diverse sentence structures and to have a lower level of writer detachment and formality of language without understanding the distinguishing features of spoken English and written English. It is recommended that they be exposed to a variety of genres of writing so as to raise their awareness of the linguistic and stylistic features of academic writing and they could improve with the help of various writing strategies.