The purpose of this paper is to investigate the impact of problem-based learning (PBL) on freshmen engineering students' beliefs about physics and physics learning (referred to as epistemological beliefs) and conceptual understanding of physics. The multiple-choice test of energy and momentum concepts and the Colorado learning attitudes about science survey were used to collect the data. The sample consisted of 142 students enrolled in the PBL and traditional lecture classes in the engineering faculty of a state university in Turkey. The analyses showed that the PBL group obtained significantly higher conceptual learning gains than the traditional group and the change (improvement) in the PBL group students' beliefs from the pre- to post test were significantly larger than that of the traditional group. The results revealed that beliefs were correlated with conceptual understanding. Suggestions are presented regarding the implementation of the PBL approach. (Contains 5 tables and 3 figures.)