One of the fundamental sciences to be learned is mathematics. In Indonesia, students learn mathematics even from preschool up to higher education. Nevertheless, based on the international studies, the Programme for International Student Assessment (PISA) and the Trends in International Mathematics and Science Study (TIMMS) in 2015, mathematics achievement of Indonesian students are still ranked at the bottom. Mathematics achievement is closely related to the cognitive domain, which not only requires cognition but also involves metacognition. This research aimed to further examine the relationship between the use of metacognitive strategy on completion of mathematics word problems and mathematics achievement among 3 rd-grade elementary students. Participants in this research amounted to 94 students from two elementary schools (one public school and one private school). The results of this research indicated that there was a significant positive relationship between the use of metacognitive strategy and mathematics achievement among 3 rd-grade elementary students (r = 0.35, p < 0.01, 1-tailed). Therefore, students who used metacognitive strategy in mathematics learning showed good mathematics achievement as well.