This study investigates the velocity and temperature distributions of the thermally fully developed electroosmotic flow through a rectangular microchannel. Based on linearized Poisson-Boltzmann (PB) equation, Navier-Stokes equation and thermally fully developed energy equation, analytical solutions of normalized velocity, temperature and Nusselt number are derived. They greatly depend on the ratio K of characteristic scale of the rectangular microchannel to Debye length, width to height ratio alpha and Joule heating to heat flux ratio S. By numerical computation, we found that for prescribed electrokinetic width K, increased S yields greater temperature. For small K, the variations of the temperature Dare larger than those of large K. The dependence of temperature on S is more significant for a small K, while at a larger K the temperature profiles are almost identical. In addition, we illustrate the Nusselt number Nu variations with S, alpha and K. (C) 2012 Elsevier Ltd. All rights reserved.