This thesis analyzed errors in the writing samples collected from 100 high school students who have learned English through the 7th Korean curriculum, in order to investigate their English grammar knowledge. The findings of this analysis are as follows. First, the rate of the whole errors was 58%. Of the causes for the errors, inter-lingual transfer (57.67%) was more prevalent than intra-lingual transfer (42.33%). Second, students with better writing skills reveal better knowledge of all the nine grammatical categories. Third, word-order use showed the highest rate of Pearson`s statistics value, implying that word order is easier to be learned than any other aspects of grammars. Finally, the comparison of students` writing of the 6th and 7th Korean curriculums showed that the rate for correct use of gerunds, infinitives, adjectives, articles, prepositions, possessives, and adverbs had decreased, with the only improvement having occurred in word-order use. This implies that the next curriculum should be designed to put more emphasis on comprehensible output.