We aimed to reveal effectiveness and weaknesses of grammar-translation method to improve ESL learners’ writing skill. We used mainly qualitative methods of trial training, documentary review and observational method. researchers conducted trial training for A2-levelled 49 learners in the period of 21 full days. To conduct trial training, we used 3 methods of teaching English grammar systematically, changing sentences based on sentences’ formula and translation from Mongolian into English. We took a level placement test at the 1^st day of training, a mid-exam at the 11^th day and final exam at the last day of the training. Then, compared learners’ outcomes in writing skill. For level placement test, they got just 30 percent and this result has been improved to 78.8 percent. But the learners’ performance dropped to 69.2 percent because of more complex sentences. Grammar-translation method is useful for simple sentences more than extended or composite sentences.