This study aimed to investigate the relative effectiveness of consciousness-raising tasks and structure-based production tasks in comparison with the traditional teaching in learning comparative and superlative forms, following a task-based approach to teaching English grammar. To this end, from among 82 female elementary-level high school students having taken a Solutions Placement Test (2010), 72 students being homogeneous regarding their language proficiency were assigned to one control group experiencing traditional grammar learning and two experimental groups that were instructed using consciousness-raising tasks and structure-based production tasks. The study was a quasi-experimental one following a pre-test post-test control group design. All groups took a multiple-choice researcher-made pre-test measuring their knowledge of comparative and superlative forms at the outset, and after six weeks, the three groups took part in the corresponding post-test. At last, the results of ANCOVA indicated that consciousness-raising task group had the best grammar performance, but no significant differences were found between the participants in structure-based production tasks and traditional teaching. Therefore, it is recommended that other EFL teachers consider consciousness-raising tasks as an option in teaching comparative and superlative forms in their high school classes.