Abstract: The Z-direction impact energies of a series of 10Ni5CrMoV steels with different plate thicknesses were tested. The fracture morphologies were observed by SEM, and microstructures were analyzed by TEM. The results show that with the increasing of the plate thickness, the number and the size of precipited phase of M₂₃C₆ in the core of the steel plate increase, which leads to the decreasing of Z-direction impact energy.