The thermal diffusivity of amorphous polymers was investigated by the newly developed a.c. joule heating technique in the practical temperature range including the melt state, glass transition and glassy state. The temperature dependence of thermal diffusivity was measured both in the glassy state and the melt state and its discontinuous change at the glass transition was clearly observed. Delta alpha, defined as the change of thermal diffusivity during the glass transition, was closely related to the glass transition temperatures and to the change in heat capacity, Delta C-p. It is confirmed that the thermal diffusivity is one of the basic thermophysical properties which can connect the chemical structure and molecular weight of polymer to the glass transition.