Abstract: The effects of the size, distribution and chemical composition of inclusions on nucleation of acicular ferrite in deposited metals of micro-alloyed steel were studied.The results show that the inclusions as nuclei of acicular ferrite are chemical heterogeneous compounds containing various elements.Most of them are within 0.2-0.6 μm in diameter.The inclusions reduce the energy barrier to nucleation by acting as high-energy inert substrates, promoting the nucleation of acicular ferrite.The primary acicular ferrite plates initially nucleate at intragranular inclusions and then many fine interlocked acicular ferrite grains nucleate sympathetically and grow from the primary plates.