A dual 137 GHz heterodyne radiometer system was used to study grooved nuclear grade graphite (SGL Group NBG17) inside an electric furnace from room temperature to 1250°C. The millimeter wave radiometer views were collinear with the electric field of one polarized parallel, and the other perpendicular, to the grooves. The anisotropic emissivity was readily detected for 100 μm wide grooves of various depths with a spacing period of 0.76 mm. The emissivity in the 500 – 1250°C temperature range was found to be 5.1 ± 0.5% when the E-field was parallel to the grooves and a factor of 2 – 4 higher, depending on groove depth, in the perpendicular direction. The parallel surface emissivity which was identical to ungrooved surface emissivity corresponded to a 137 GHz surface resistance of 5.3 Ohms, which is about 2.5 times higher than the value predicted from frequency scaling dc surface resistance. The perpendicular emissivity had a modulation with groove depth at odd integral multiples of ¼λ, predicted by electromagnetic finite difference time domain analysis.