The monotonic heating regime method for determination of thermal diffusivity is based on the analysis of an unsteady-state (stabilised) thermal process characterised by an independence of the space-time temperature distribution on initial conditions. At the first kind of the monotonic regime a sample of simple geometry is heated / cooled at constant ambient temperature. The determination of thermal diffusivity requires determination the rate of a temperature change and simultaneous determination the first eigenvalue. The eigenvalue is found from a relationship between the synchronous temperatures measured at the surface and at the middle of specimen, which is expressed by eigenfunctions in the analytical solution. According to a characteristic equation the first eigenvalue is a function of the Biot number defined by surface heat transfer coefficient and thermal conductivity of an analysed material. Knowing the surface heat transfer coefficient and the first eigenvalue the thermal conductivity can be determined. The surface heat transport coefficient during the monotonic regime can be determined by the continuous measurement of long-wave radiation heat flow and the photoelectric measurement of the air refractive index gradient in a boundary layer. The obtained eigenvalues and corresponding surface heat transfer coefficient values enable to determine thermal conductivity of the analysed sample together with its thermal diffusivity, during a monotonic heating regime.