In this paper the authors report thennal conductivity and piezoresistivity measurements of top-down fabricated highly boron doped (N A = 1.5 x 10 exponent 19 cm-3) suspended Si nanowires. These measurements were perfonned in a cryogenic probe station respectively by using the 3 omega method and by in situ application of a longitudinal tensile stress to the nanowire under test with a direct four point bending of the Si nanowire die. Nanowires investigated have a thickness of 160 nm, a width in the 80-260 nm range and a length in the 2.5-5.2 um range. the authors found that for these geometries, thennal conduction still obeys Fourier's law and that, as expected, the thennal conductivity is largely reduced when the nanowires width is shrunk, but, to a lower extent than published values for nanowires grown by vapor-liquid-solid (VLS) processes. While a large giant piezoresistance effect was evidenced by various authors when a static stress is applied, the authors only observed a limited nanowire size dependence of the piezoresistivity in the experiments where a dynamical mechanical loading is applied. This confinns that the giant piezoresistance effect in unbiased Si nanowires is not an intrinsic bulk effect but is dominated by surface related effects in agreement with the piezopinch effect model.