Silicon nanowires (SiNWs) field effect transistors (FETs) with a surrounding gate may enable further scaling of CMOS devices due to their thin body and cylindrical geometry. Of fundamental importance is the method in which contacts to the nanowire channel are being formed. One promising method utilizes thermally activated axial intrusion of nickel silicides into the SiNW from pre-patterned Ni reservoirs located at both ends of the wires. Such intrusion of Ni-silicide involves different thermally activated processes such as volume, surface and interface diffusion of Ni, each being characterized by a certain time, temperature and SiNW diameter dependencies. Up to date a comprehensive understanding of these processes is still missing. In the present work these dependencies were investigated during axial growth of nickel silicide in SiNWs for a temperature range of 300-440°C and wire diameters of 30-60nm. Nickel electrodes were deposited on randomly dispersed SiNWs followed by rapid thermal annealing processes at different temperatures and for different times. Silicide intrusions were investigated by atomic force microscopy and scanning electron microscopy. The following results were analyzed in the framework of diffusion phase formation model that we have published recently. The main part of the intrusion consists of monosilicide NiSi, as was confirmed by measuring the electrical resistance of the wire after full silicidation. Nickel silicide intrusion length, L, showed a typical diffusion dependence on the annealing time, t, and temperature, T: L= kt<sup>1/2</sup>exp(-<I>Q</I>/2<I>k<sub>B</sub>T</I>), where <I>Q</I> is the diffusion activation energy, and k is proportionality coefficient depending on the wire diameter. We have studied the dependence of k on the wire diameter, and the activation energy for the growth was found to be ~1.7 eV which is typical for nickel interface diffusion processes in nickel silicides. It was concluded that the growth of nickel silicide intrusion is controlled by surface diffusion of nickel along the outer surface of the silicide formed and not by nickel diffusion through the silicide bulk.