The deformation of the Earth’s continental crust depends largely on the mechanical properties of quartz and feldspar, including both brittle and ductile mechanisms. However, viscoplastic deformation of these mineral phases is difficult to achieve in laboratory experiments at temperatures relevant to the upper crust without high confining pressures to suppress microcracking and brittle fracture. In this study we use instrumented nanoindentation to determine the plastic yield strength of quartz, and orthoclase and plagioclase feldspars at temperatures of 23 – 500 °C and at strain rates of ~10-2 s-1. The specimen investigated here is a medium-grained granite from southwestern Rhode Island, grains of which were characterized and oriented using scanning electron microscopy and electron backscatter diffraction (EBSD). Indentation hardness and modulus of oriented grains was measured directly using a diamond Berkovich nanoindenter; these quantities were then used to calculate yield stresses through several existing theoretical and numerical models. The calculated yield stresses are fit to constitutive flow laws for low-temperature plasticity. We find that at these deformation conditions yield stresses are significantly lower than the stresses predicted by the extrapolation of previous flow laws from higher temperatures. We also find that, in contrast to most observations from nature, quartz is systematically stronger than both feldspars.