The purpose of this study is to analyze grammatical errors in English writing of elementary students and to compare the aspects of grammatical errors of students amongst three different groups in an attempt to explore effective ways of teaching grammar and writing. The grammatical errors were classified into four categories of addition, omission, substitution, and word order. Within each category, the errors were again classified into ten sub-items which include verbs, articles, nouns, pronouns, adjectives, adverbs, prepositions, conjunctions, punctuation, and capital/small letters. The number of errors and its proportion from certain sub-items were calculated to identify the frequent grammatical errors. The results of the study suggested that the most frequent grammatical errors were verb errors, which were mainly the third person singular verb errors, errors of tense, overgenerated be-verb errors. The next frequent type of errors was the omission of articles. Errors of punctuation and classification of capital and small letters also appeared frequently. In addition, the aspects of errors occurred in three leveled groups indicated similar results but showed slight differences in specific error patterns. The findings of this study are to understand the difficulties of learning English grammar rules and discover the effective ways of teaching grammar that can help the students overcome the difficulties with learning grammar aspects.