An effective instructional approach in elementary mathematics education has a significant role in the understanding of mathematical concepts and overall student achievement. We point out the characteristics, significance and effects that may be achieved in mathematics education through the application of project-based learning. The aim of the study was to examine the effects of project-based learning on student achievement in lower elementary mathematics education and examine whether the project-based model was equally acceptable to students with different marks. We arranged quasi-experimental research instruction (experiment with parallel groups), on a sample (N = 147) in order to examine whether an instructional approach based on the principles of project-based work would achieve better effects of learning and student achievement compared to the usual way of learning implemented in mathematics education. Results of the final measurements show that students in the experimental group, who worked according to the model of project-based work, achieved better compared to students who worked in the usual way. With this study we have shown that project-based instruction has significant effects on student achievement in lower elementary mathematics education, and that it can undoubtedly contribute to the methodological empowerment of teachers in their teaching practice.