The mechanisms of acicular ferrite formation on non-metallic inclusion have been studied in high strength ship plate steel. The effect of inclusion size and type on the formation of acicular ferrite was studied by metallographic microscope and SEM-EDS. The inclusion composition was analyzed by Raman spectra. Experimental results showed that all the inclusions were nearly spherical in shape. Inclusion size was mainly from 1 µm to 4 µm in high strength ship plate steel. With increase of the inclusion size, the acicular ferrite nucleating rate increased. The probability of inducing nucleation increased gradually with the inclusion size increasing. The inclusions had an appropriate size to become nucleation center for acicular ferrite. This optimal size was about 3 µm. The inclusion was composed by Ti, Mn, S, Si, O, Al and Mg. The composition of Ti–O–Mn inclusion was Ti₂O₃ and MnS, but no MnTiO₃. The surface of inclusion as an inert surface played an important role in nucleating of acicular ferrite. The lower low mismatch was also the important factors to promote the nucleation of acicular ferrite. Based on the two mechanisms, it would make it easier for acicular ferrite to nucleate.