Abstract: A transient one-dimensional coupled conduction-radiation heat transfer model was developed for thermal diffusivity measurement of semi-transparent materials by the flash method. On the basis of the control volume method, the discrete ordinate method (DOM) was used to solve the radiation heat transfer equation of the model. It showed that the numerical results were agreed well with those by the thermal quadrupoles method (TQM). DOM was a little bit time-consuming but was available for a non-linear physical property problem. Only a constant physical property problem can TQM solved, although it is faster than DOM. The sample's backside temperature response was analyzed for various sample thickness, absorption coefficient, radiative boundary condition and heat loss.