The goal of the present study is to investigate the impact of procedural and conceptual teaching on students' mathematical performance over time. For this purpose, implicit differentiation and integration by parts were taught in two different ways to first-year university students of two different calculus courses: one course with Conceptual Teaching (CT) and the other course with Procedural Teaching (PT). In the CT class, the lecturer taught mathematical concepts conceptually and helped students to understand the reasons behind techniques and formulas. In the PT class, the lecturer helped students to solve problems without necessarily knowing the reasons for each step of solving problems. Both groups took a same test at the end of these lessons. The results of the test showed that there is no significant difference between the performance of the two groups. After three months, another test including the same topics had been taken by the two groups. The results of this test showed that the CT group had a better performance compared to the PT group. The findings suggest that conceptual understanding could create more sustainability in learning mathematics.