Perceived as one fundamental element in language learning, grammar is reckoned important in ESL writing. ESL learners need to master the knowledge of how to transfer grammatical concepts into their ESL composition. However, Malaysian learners of English often repeat errors in writing which they cannot even recognize. The current study draws attention to the occurrences of language errors and examines their sources in Malaysian ESL learners’ writing. 40 students in their Semester 2 of diploma level were selected to participate in the study. Each student was to produce two essays of 100-word length. Each essay was first examined before language errors were identified and coded based on the parts of speech: Nouns, Verbs, Adjectives, Adverbs, Articles, Pronouns, Prepositions, Adverbs, Conjunctions and Determiners. For each type of errors, the sources were categorized based on interlingual and intralingual sources. 258 errors were identified with the most frequent language errors produced were verb errors while the least were determiner errors. The result revealed that the most dominant errors were caused by intralingual sources. This study would greatly help teachers to establish better curriculum and select materials to facilitate students in learning English and develop them as proficient learners who can self-correct language errors.