For the evaluation of thermoelectric properties in silicon nanowires (SiNWs), thermoelectric test structures are manufactured, including 50-nm-wide n- and p-type SiNWs, micro-heater and temperature sensors using a conventional lithography method on 8 in. silicon wafer. For the optimization of thermoelectric properties in SiNWs, we have evaluated Seebeck coefficients and power factors of n- and p-type SiNWs by varying the nanowire length 10, 40 mu m and temperature (from 310 to 450 K). The results show that the maximum Seebeck coefficients and power factors are and for long p-type and long n-type SiNWs, respectively. The contribution of phonon-drag effect to thermoelectric power is discussed in the highly doped SiNWs.