The variation of microstructure and their effect on the mechanical properties of heavy plate for ship hull treated by quenching in full austenite region at 910°C once or twice and tempering, or quenching in full austenite region and quenching in intercritical austenite region at 840°C again and tempering were studied with SEM and TEM. It was found that the strength and low temperature toughness of the steel were higher after double quenching in full austenite region and tempering than the steel which was quenched once and tempering, because smaller bainite inter-lath spacing, finer grains were obtained. The steel treated by intercritical quenching at 840°C after quenching in full austenite region and tempering showed the best combination of mechanical properties. This can be attributed to comprehensive effect of the uniform distribution of ferrite and bainite, stronger grain refining effect caused by formation of ferrite.