This study investigated the effect of the metacognitive instruction in which students' epistemic cognitions were explicitly addressed, on tenth-grade students' conceptual understandings regarding force and motion. The participants of the study included 107 (49 female, 58 male) tenth-grade students at two public high schools. A quasi-experimental design was employed. Two intact classes of each school were randomly assigned to the experimental and control groups. The Force and Motion Conceptual Tests I and II were administered to assess the students' conceptual understandings in force and motion. A survey was applied to probe the students' prior epistemic cognitions in physics. The results of the study revealed that the metacognitive instruction was more effective than the expository teaching in terms of promoting students' conceptual understandings. A statistically significant interaction between the treatment and the students' prior epistemic cognitions was also observed. The students with higher pre-epistemic cognitions got higher conceptual scores in the experimental group whereas the conceptual development of the control group students was independent of their pre-epistemic cognitions.