We report here both heterogeneous and homogeneous nucleation of epitaxial silicide of NiSi2 in Si nanowires grown in  direction by in situ observation in high-resolution transmission electron microscopy (TEM). Owing to the excellent lattice match between Si and NiSi2, a giant epitaxial step of 2-8 nm wide forms at the interface between Si and NiSi2 during the growth of the latter. The step formation results in two silicide/Si interfaces parallel to each other in TEM observations. Heterogeneous nucleation of NiSi2 occurs at the intersection where the step meets the interfaces. However, we have also observed the epitaxial growth of NiSi 2 having a single interface with the Si, i.e., without a giant step. Homogeneous nucleation of NiSi2 occurs on the single interface. Incubation time of heterogeneous nucleation of NiSi2 has been measured by high-resolution video to be much shorter than that of homogeneous nucleation. The overall growth rate of NiSi2 for the case of heterogeneous nucleation is faster than that for the case of homogeneous nucleation. Kinetic analysis of both types of nucleation is presented for a direct comparison in order to have a better understanding of the nucleation events.