2.000) and (3,666 > 2,000) , so that the research hypothesis is accepted. From the analysis of the result above, it can be concluded that English Grammar Mastery has positive effect on writing ability of the eighth year students of Junior High School at SMP Negeri 4 Jatibarang Kabupaten Brebes in Academic Year 2009/2010. The writer would like to give suggestion to the Eglish teacher and the students, English teacher must prepare the material for teaching. For the students should have a lot of exercize for grammar and writing." />