The present study aims to explore Turkish EFL students' major writing difficulties by analyzing the frequent writing errors in academic essays. Accordingly, the study examined errors in a corpus of 150 academic essays written by Turkish EFL students studying at the Department of English Language and Literature at a public university in Turkey. The essays were written on assigned topics as take home exam papers or assignments in the context of a first year academic writing course. The corpus consisted of essays of various lengths ranging from 500 word essays to 1500 word essays. The essays were compiled into a corpus and analyzed by using a concordance program. The essays were also checked for plagiarism using the online plagiarism detection software and plagiarized essays were excluded from the analysis. Errors were classified by using an error classification system which was organized according to lexico-grammatical categories. The resulting categories consisted of mostly syntactic and lexical categories of error but academic style errors were considered as well. As a result of the analysis, in terms of error categories, the most frequent errors were observed in the verb related error categories. When considered individually, the most frequent errors were observed in noun modification and were mostly interference related.