  We present an asymptotic preserving method for the radiative transfer
equations in the framework of PN method. An implicit and explicit method is
proposed to solve the P N system based on the order analysis of the expansion
coefficients of the distribution function. The order of each coefficient
expanded in the Knudsen number is found through the process of Maxwellian
iteration, and the coefficients of high order are treated explicitly while that
of low order are treated implicitly in each equation of P N system. Energy
inequality is proved for this numerical scheme. Several numerical examples
validate this new AP scheme in both optical thick and thin regions.
