The C curve of supercooled austenite of plastic mould steel P20B was investigated with the DIL805A dilatometric instrument. The austenitizing temperature was between 850 ℃ and 1 000 ℃ and the isothermal temperature between 640 ℃ and 720 ℃. We compared austenite grain size at different austenitizing temperatures and analyzed the second phase which obtained from the electrolytic extraction procedure. The results show that at 850~920 ℃ in the temperature range when the austenitizing temperature is increased, the grains grow slightly, the content of B increased significantly to make the C curve strongly shift right. At 920~1 000 ℃ in the temperature range when the austenitizing temperature is increased, the grains grow significantly, and the content of B no longer increases so hardenability of P20B steel does not increased. 