the purpose of this thesis was to investigate how differently metacognitive, cognitive, and social/affective strategies affect l2 learners’ reading comprehension. to this end, the study employed a quasi-experimental design with a placement test as a proficiency test to find the homogeneity of groups. three classes were randomly selected as the experimental groups (n =90), and each class was taught by one of the learning strategies according to brown (2000) taxonomy (metacognitive, cognitive, and social/affective strategies) respectively. all groups participated in six sessions of the task of developing familiarity with each strategy. each group received one main strategy and then, according to oxford (1990) training model, the researcher taught and employed those strategies in reading comprehension texts. learners’ improvement was measured during the six sessions of teaching and employing those strategies in responding to reading questions. one-way anovas with post hoc comparisons indicated that metacognitive group significantly outperformed the other groups and, so metacognitive strategies are more effective in promoting l2 learners’ reading comprehension. these findings suggest that employing metacognitive strategies are the most helpful factor and also a positive predictor compared to other strategies in improving learners’ awareness to answer reading comprehension tests. it can also be concluded that cognitive strategies are the least useful strategies compared to social/affective and metacognitive strategies in terms of their effectiveness.