In this study, the effects of wall slip and streaming potential on liquid flow and heat transfer in planar microchannels with imposed heat flux are numerically investigated. Electrical potential of the electrical double layer, liquid flow and thermal characteristics is determined using the Poisson-Boltzmann, the modified Navier–Stokes and the energy equations, respectively. The analytical solution for pressure-driven electrokinetic flows in microchannels is obtained without introducing the Debye–Huckel approximation. The results reveal that the streaming potential effect retards the liquid flow and leads to an increase in the temperature of electrolyte solution, thereby decreasing the heat transfer rate. On the contrary, the wall slip effect tends to increase the flow velocity and hence enhances the heat transfer. When the above two effects appear in microchannels simultaneously, the wall slip assists the streaming potential to retard the flow. Furthermore, they counteract each other when the zeta potential is large enough.