This study aimed to analyse the effect of guided inquiry learning in improving metacognitive skill of elementary school students in fractional materials. This type of research was a mixed of quantitative and qualitative methods. The subjects of this study consisted of 55 fifth grade students. Two learning models -- the guided inquiry learning and conventional learning -- were compared. The students' metacognitive skills were measured through fractional material problem solving tests. The quantitative data analysis used descriptive and inferential statistical tests, while the qualitative data were collected through unstructured interviews. Based on the data analysis, it was found that the sig (2-tailed) t-test from the independent post-test t-test was 0.00 (p = <0.05), indicating that there was significant difference. This shows that the students' metacognitive skill of the two classes were different in solving fraction problems after the application of guided inquiry. Thus, it can be concluded that there is a significant influence on the application of guided inquiry learning in improving students' metacognitive skill in solving fraction problems.