We investigated the phase transformation and thermal stability of Ni silicides formed in Ni/Si and Ni0.95Ta0.05/Si systems. The sheet resistance values of the silicide in the Ni0.95Ta0.05/Si system were lower than those in Ni/Si system at any temperature. The enhancement of thermal stability is closely related to the phase transformation occurred during post heat-treatment. Microstructure of the phases formed by reaction was investigated by analytical electron microscopy (AEM) and the phase identification of Ni silicide was carried out using convergent beam electron diffraction (CBED) technique. It was found that a Ta rich layer formed on the top of the Ni silicide layer and small amount of Ta dissolved into the silicide layer. By addition of Ta atoms, phase transformation from NiSi to NiSi2 is retarded and thermal stability of Ni silicide is improved.