Recent technological advances have raised the issue of how to integrate technology into language classrooms. With the emphasis on student-centred learning, the applications of the computer have enhanced not only the input but also the output in language teaching and learning. This study is an attempt to find out the role of a web-based grammar instruction system in students' grammar achievement and students' perceptions about the system. This web-based grammar instruction was provided via an online grammar lab serves as a blend of the coursebook, online, and mobile sources. It has three level series; however, for the present study its intermediate level was used. The study was conducted at the Department of English Language and Literature at a major university in north-eastern Turkey throughout the academic year 2012-2013. English-majoring students with access to the online grammar lab were assigned as the experimental group (n = 73), and the students who take grammar instruction in traditional whole-class environment as control group (n = 57). While four achievement exams served as the quantitative data gathering instrument of this one-year quasi-experimental study, an open-ended questionnaire was conducted with 53 voluntary students so as to gain more insights into the use of web-based grammar instruction.