Seamless tubular components, such as stepped tubes, conical tubes and box shape tubes, are mainly produced in tube hydroforming process. In forming the components in this process, complete filling the die corners is very difficult. In this paper, the mechanism of improvement of die corner filling in a proposed tube hydroforming die was investigated. The FEM simulation results showed that the material flow and stress distribution could theoretically clarify the die corner filling in the proposed die. Also, the comparison of the die corner filling between the new die and a conventional die was explained. In order to verify the simulation results, some experiments were performed.