Silicidation of Si Nanowires (Si NWs) has been investigated in terms of the encroachment length of Ni suicide from the edge of exposed Si NWs. Ni suicides were formed by the reaction of Si NW and Ni layer by low temperature RTA processes. The time dependence of the encroachment length revealed that this phenomenon is governed by the diffusion of species. Activation energy of the diffusion on changing NW width from 10 nm to 23nm were estimated to be 1.48-1.60 eV using 30-nm-thick (100) SOI substrate. The estimated activation energy using 30nm-thick (100) SOI substrate are larger than 1.23 eV of (100) bulk Si (1). We also investigate Ni silicidation of Si NW using two step annealing process. The encroachment length of Ni suicide is dramatically suppressed for the two step annealing process.