In recent years, the hydroforming due to reduction in weight and increase in strength of automotive parts has attracted attention of the automotive industry. Thus, the knowledge of the process to produce parts with desired quality and accuracy has been growing. One of the effective methods in improving the formability in the hydroforming is known as pulsating hydroforming. However, the reason of improvement of formability in pulsating hydroforming is still unclear for the closed-die hydroforming of tubes. In this study, the pulsating hydroforming of tube with box die was simulated by the finite element method to examine the effect of pulsating pressure on improvement of formability. In addition, the calculated results are compared with experimental results and it is shown that the improvement of the formability occurs before contact of the tube and die in expansion zone of the tube. Thus, it can be obtained from the results, that the main reason of improvement of the formability is due to the deformation behavior before start of contact of the tube and die.