Rotary kilns are used ubiquitously in the chemical and metallurgical industries. The mechanism of heat transfer in a rotary kiln is discussed in this paper, in which the effect of rotation is considered in determining heat transfer coefficients. In particular, an extended penetration theory is successfully developed to describe the heat transfer coefficient of the covered wall to the bulk solid in a rotary kiln, i.e., = ((X)d(p)/k(p) + 0.5/root 2k(b)p(b)c(pb)n/Phi(0))(-1) (0.096 < (X) < 0.198) A one-dimensional axial heat transfer model for an internally heated rotary kiln has been developed. Both predicted temperature profiles and heat transfer fluxes agree well with the experimental data of Barr et al. The simulated results are used to successfully explain for the first time the coupling phenomenon of the bulk bed and covered wall temperatures discussed in previous publications.