When two nanowires cross each other, they form a point contact. Point contact reaction between a nano metal wire and a nano Si wire has been studied by using ultra-high vacuum and high resolution transmission electron microscopy. Axel epitaxial growth of nano silicides of NiSi and CoSi2 in nanowires of Si has been observed. The nucleation stage and stepwise growth stage of the reactive epitaxial growth of nano silicide on nano Si have been measured. A repeating event of homogeneous nucleation has been found, which enables us to estimate the number of molecules in a critical nucleus to be about 10 using the Zeldovich factor. A comparison to heterogeneous nucleation will be made. The nucleation-controlled or supply-controlled growth mode of point contact reactions is different from the well-known diffusion-controlled and interfacial-reaction-controlled modes of growth in thin film and bulk samples.