In this paper, the mechanism of improvement of formability in pulsating hydroforming of T-shape tubes is investigated by the finite element simulation and experiment. It is shown that local thinning was prevented by oscillating the internal pressure, because the protrusion is formed gradually by the prevention of sharp bulging. In the hydroforming, for the pulsating pressure, several steps occur in variations in wall thickness, and thus, the thickness of the tube increases, whereas, for the peak pressure, the thickness is reduced continuously. Moreover, the effects of the amplitude and the number of cycles of pressure per unit punch, the stroke on formability and the corner filling are examined. It is shown that the small number of cycles of pressure and large amplitudes improve the formability, whereas, a large number of cycles of pressure and small amplitudes increase the die corner filling and shape accuracy as well.