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We have simulated the electronic states and the piezoresistive effect response to mechanical strain in single-crystal silicon nanowires (SiNWs) with hydrogen termination by using first-principles calculations of model structures with various wire orientations. Based on our original idea for a small amount of carrier occupation, the carrier conductivity along the wire axis has been calculated in terms of band carrier densities and their corresponding effective masses derived from the one-dimensional first-principles band diagram. In the hydrogen-terminated <001> SiNW model, the uniaxial tensile stress to the longitudinal direction causes a sharp drop in the band energy of the highest valence-band (VB) subband, leading to the redistribution of holes to other VB subbands with a huge hole effective mass. The sudden change in the hole occupation with the increase in effective mass will bring a drastic decrease in the hole conductivity. We have obtained a giant longitudinal piezoresistance coefficient for the p-doped <001> SiNW model, and it is expected that p-doped <001> SiNW without dangling bonds will be one of the most suitable candidates for NEMS piezoresistors due to its giant piezoresistivity. On the contrary, the hole conductivity for the p-doped <111> SiNW depends only on the hole mobility of the highest VB subband. As a result, the longitudinal and transverse piezoresistance coefficients for p-type <111> SiNW without dangling bonds are very small.