High-resolution thermal-conductivity and heat-capacity studies on the same sample have been carried out near the smectic-A-hexatic-B transition of n-hexyl-4' -n-pentyloxybiphenyl-4-carboxylate (65OBC). A power-law fitting with background contribution to the thermal conductivity anomaly leads to a dynamic critical exponent a=0.51 and a ratio of critical amplitudes +-=0.93. We will discuss our results on the basis of a dynamic scaling. The thermal diffusivity shows a pronounced drop near the transition, which clearly demonstrates critical slowing down of the thermal fluctuations. mal fluctuations.