In this paper, the characteristics of rotary forging deformation and their mechanic analysis are briefly described; meanwhile the four kinds of defects caused generally inthe deformation are introduced: chevron crack in the thin workpiece, peripheral crack in the flange with a rod, instability and overlapping, and not thoroughly forging. Then, it is pointed out that the defects such as crazing and pressed-subsidence are frequently caused in the centre of an upper die, because of the particularities of the rotary fbrging technology: there is a small contact area throughout between the upper die and the workpiece and the pressure is high, and the upper die suffers from the effect of thermal fatigue during hot rotary forging. Finally, the measures to prevent the defects and the methods for impoving the service life of the die are proposed. For thick workpieces and flanges with a rod, the feed per rotation should be properly controlled; for thin workpieces, the feed per rotation should be increased. In cold rotary forging of flanges with a rod, the surface quality of blanks should be considered and the die-workpiece clearance should be controlled so that the defects of cracking, instablity, overlapping and so on can be decreased.