EFL students’ grammatical ability has been often discussed in apposition with writing ability. The role of grammar in writing has been extensively argued, yet not reaching a consensus conclusion. The major reason is due to how grammatical and writing abilities are defined as well as the variables of the design and assessment of the tests. Thus, this study proposed a theoretical model for each ability. Based on the models, this study tried to examine whether grammar instruction could promote the students’ grammatical ability, thereby further helping their writing. In this study, the test, divided into a grammar subset and a writing subset, was administered to ten senior high school students in a lowerintermediate English class at a cram school in Southern Taiwan. The grammar subtest and the writing subtest comprised 40% and 60% of the whole test respectively. The results indicated that most students’ grammar subtests outperformed their writing subtests. The finding implied that there was no strong relationship between the knowledge of grammar and usage among lower-intermediate learners. In addition, some suggestions were provided for improving instruction and both grammar and writing subtests. EFL 學生文法能力常與寫作能力被相提並論，文法在寫作中之角色一直被廣泛爭論卻仍未達一致結論，主因是如何定義文法寫作，及其能力之測驗設計和評量等變數有關。故本研究設計評量文法及寫作能力模型，以檢視文法指導是否提升學生文法能力並進而幫助其寫作。本研究中測驗含文法及寫作兩項目，文法及寫作分數比各佔40%及60%，被執行於南台灣補習班英文課中十位中低英文能力之高中生。結果顯示大部分學生文法測驗表現比寫作好。此發現暗示文法知識及其使用，在中低英文能力學生身上無明顯關聯。一些建議也被提出，以供改進教學與文法寫作測驗設計。
