In this study, we explored the critical thinking of 47 eleventh-grade students in a mathematics problem-based learning (PBL) classroom in November 2016. A critical thinking test was used along with classroom observations to gather the critical thinking data in five dimensions according to the Association of American Colleges & Universities (2009). The findings indicate that students' critical thinking scores in all dimensions are at an average level. The students demonstrated strength in explaining issues and analyzing influence of context and assumptions. However, students had greater difficulty in stating their positions and drawing conclusions.