Anal. of early failed 4Cr3Mo2MnVNbB steel warm-forging die for surgical instruments was conducted by means of chem. composition anal., hardness examination, metallog. anal. and fractog. The results show that there exists more serious dendrite segregation in the steel, which leads to uneven microstructure. During warm-forging, crack initiates on surface of the die where the sustained tensile stress is higher than the steel strength and propagates to core of the die along the lines of dendrite segregation and is rapidly developed with the increase of thermal fatigue times, resulting in earlier fracture of the die.