The suspension system is an essential component that ensures a car moves smoothly and stably on all road conditions. For heavy-duty vehicles, the leaf spring is one of the crucial parts that help the vehicle move smoothly and stably. This study used the finite element analysis method to simulate and calculate the leaf spring on the Hyundai County bus, a very popular type of passenger vehicle in Vietnam. A new leaf spring set was designed with varying thickness and deflection, simulating with four materials: Steel, EN45 Steel, Kevlar Epoxy, and Composite materials. The results showed that the displacement and stress of Composite materials were much higher than steel. The smaller the deflection, the lower the stress and displacement, indicating better durability of the spring. However, this results in reduced ride comfort. The leaf spring with a 63 mm deflection had the lowest stress, while the one with a 113 mm deflection was suitable for most materials, with no material failure in this case. The leaf spring with a 157 mm deflection was suitable for EN45 Steel, E-glass, and Kevlar Epoxy materials. The best-balanced material in this study was Kevlar Epoxy with a 157 mm deflection, providing good load-bearing capacity, ride comfort, and vehicle stability.