We have investigated 'hexa- n -alcanoyloxytruxene' HATXC H which exhibits a disk-like thermotropic nematic phase between 65 and 84 C. Using a light-beating technique we have measured the orientational diffusivities D a correspond respectively to twist and bend deformations. D twist increases monotonically as temperature increases, while D remains constant. This result means that the curvature bend elastic constant K changes more rapidly than the viscosity eta 22 in the same ratio. This means also that in the discotic thermotropic nematic phase, when temperature increases the effective hydrodynamic object, in orientational fluctuation, changes shape to be disk-like rather than rod-like. 9 19 nd D for several temperatures, which twist bend, while K and eta change twist 33 bend