In this paper, we first propose an improved CVD-WSix metal gate suitable for use with nMOSFETs. Work function of CVD-WSi3.9 gate estimated from C-V measurements was 4.3eV. The nMOSFET using CVD-WSi3.9 gate electrode showed that Vth variation of L/W=1 μm/10μm nMOSFETs can be suppressed to be lower than 8mV in 22chip. In CVD-WSi3.9 gate MOSFETs with gate length of 50nm, a drive current of 636μA/μm was achieved for off-state leakage current of 35nA/μm at 1.0V of power supply voltage. By using CVD-WSi3.9 gate electrode, highly reliable metal gate nMOSFETs can be realized.