Ni silicide films were formed on both heavily doped n+ and p+ Si substrates at various temperatures ranged from 200°C to 950°C and its electrical and structural properties were studied. It was found that the phase transition temperature from NiSi phase to NiSi2 phase depend on substrate types; 900°C for NiSi film on n+ Si substrate and 750°C for NiSi film on p+ Si substrate, respectively. It was also found that agglomerations of Ni silicide films on n+ Si substrates begin to occur at the annealing temperatures around 600°C while there is no agglomeration observed in Ni silicide films on p+ Si substrates up to 700°C. Obtained results show that the agglomeration is especially important factor in the process temperature dependency of the sheet resistance of Ni silicides formed on n+ Si substrates. It was also demonstrated that the agglomeration can be suppressed by an inserted thin Boron layer at the interface of Ni/Si.