The study aimed to identify how an English native speaker and a highly proficient non-native speaker rated the comprehensibility of English sentences written by learners who were studying non-language majors. Nine essays written by students of Pharmacy and 14 essays written by students of Accounting were rated by the two raters. They were guided by a scoring rubric which emphasized mainly on the grammaticality and comprehensibility. Their ratings classified the sentences into two major categories: sentences with mistakes that potentially hinder comprehensibility, and sentences with mistakes that completely make them incomprehensible.Mistakes that make sentences partially comprehensible include wrong word order, coma splice, wrong auxiliary, wrong form of finite verb, wrong form of active sentences, the omission of verb be, intensifier, and introductory “there”. Mistakes that completely blocks comprehensibility are fragmented phrase, wrong collocations, mismatched markers, wrong part of speech, unfinished sentences, and excessively long sentences joined by a coordinator.A number of suggestions for the teaching of grammar, writing, and vocabulary are then proposed on the basis of the findings
