This study aimed to analyze the effect of problem-based learning on metacognitive abilities in the conjecturing process of junior high school students. To reach this purpose, a mixed-methods design, which is a combination of quantitative and qualitative methods, was used. The quantitative method was used to analyze the metacognitive abilities of the students' conjecturing process, while the qualitative method was used to explore observation and interview data. The subjects of this study consisted of 60 eighth-grade students. Two learning models were compared, namely the problem-based learning model and the conventional learning model. The metacognitive abilities of students' conjecturing process were measured by a pattern generalization problem-solving test. After collecting the data and analyzing them through the independent-samples t-test, it was revealed that the PBL had a significant effect on the metacognitive abilities of students' conjecturing process in solving pattern generalization problems. Finally, based on the results, some conclusions and implications were suggested.