Flow analysis in the microchannels has recently accelerated dramatically. In this paper, numerical investigation of Joule heating effects on the electroosmotic flow through a microchannel with the trapezoidal cross-section and constant wall temperature have been presented. The energy equation for the temperature distribution, Navier–Stokes equation for the velocity distribution and a Poisson equation for the electric potential distribution have been solved by using the finite-volume method in a system curvilinear coordinates. Thermophysical properties such as the dynamic viscosity and electric conductivity vary with temperature. Results show that by increasing the Joule number, the temperature, velocity and mass flow rate increase with constant EDL number. Without considering the Joule heating effects, the increments of EDL number causes in the mass flow rate to increase, but with considering the joule heating effects, the increasing of mass flow rate continues until EDL number 15 and after that the flow rate decreases. On the other hand, when the cross-section is reduced by the increasing aspect ratio, the joule number remains constant while the mean temperature decreases.