The electroosmotic flow drives a fluid passing through microchannels by an applied electric field,which also induces heat transfers both in the fluid and channel walls because of a Joule-heating. Using the Computing Flow Dynamics(CFD) technique,a flow field and a temperature field in diverging channels are numerically investigated with a 3-D microchip model. Due to the temperature-dependent physical properties of the fluid including viscosity,relative dielectric constants,electric conductivity,and thermal conductivity,the induced temperature gradient imposes great influence on flow behaviors of channels. The results suggest that a nonuniform gradient thermal field is formed in a microchip by the Joule heating and it affects the flow field severely. This gradient thermal field has increased the flow velocity in uniform cross-section channels but can not do it similarly in diverging channels,so that the outlet velocity and volume flow rate have decreased by 16 % and 60 μl/min,respectively.Moreover,the Joule heating also weakens the pumping performance of diverging channels by decreasing the flow velocity and pressure.