We have simulated piezoresistance coefficients in single-crystal hydrogen-terminated silicon nanosheets with (001) surface orientation on the basis of first-principles calculations of model structures. The [110] uniaxial tensile stress causes band deformation, and a drastic change of conductivity can be observed for nanosheet models. We have obtained high piezoresistance coefficients for p-type nanosheet of about 1 nm thickness, pi([110]) = 174 x 10(-11) Pa-1, and pi([110]) = -139 x 10(-11) Pa-1. It is expected that p-type ultra-thin Si(001) nanosheet will be a suitable candidates for nanoscale piezoresistors due to its giant piezoresistivity.