Abstract: The hardenability curves of low alloy ultra-heavy plate steel containing boron under different quench processes were obtained by Jominy end quenching test. Under low cooling rate conditions, the microstructures of the ultra-heavy plate steel were observed by optical microscopy (OM) and the grain boundary segregation of boron was tested by auger electronic spectroscopy (AES). It is demonstrated that at the quenching temperature not higher than 920℃, the ultra-heavy plate steel has a better hardenability when the quenching temperature is higher or the holding time is properly longer, and a more perfect hardenability is got by appropriate double quenching. However, when the quenching temperature is higher than 920℃, the hardenability of the ultra-heavy plate steel deteriorates sharply by using single or double quenching.