The hot deformation behavior of as-cast TC21 alloy was investigated in the temperature range of 1000-1150 oC and the strain rate range of 0.01-10 s-1. Based on dynamic materials modeling (DMM), the processing map was established. Combined with the microstructure observation the hot deformation mechanism of TC21 alloy was worked out. The results show that the leading deformation mechanism of TC21 alloy is dynamic recovery in β region. At higher strain rate ( ≥1 s-1), the instability is attributed to flow localization, which should be avoided. The second domain (1050-1120 oC, 0.1-1 s-1), β grain boundary represents sliding, which is typical dynamic recovery. Optimal processing zone appears at the lower strain rate (0.01-0.1 s-1), which has a peak efficiency of 38%-46%, and a typical continuous recrystallization with serrated grain boundaries occurs. Thus, the first cogging should be at a higher temperature (1150 oC) to avoid cracking. Later, it should be at a lower temperature to obtain fine and homogeneous microstructure.