High heat flux is very dangerous for electronic heat transfer, such as IGBT (Insulated Gate Bipolar Transistor) cooling. In order to explore and master the heat transfer and hydraulic characteristics for IGBT cooling, experiments have been carried out to study the situation mentioned above in a flat plate heat sink, which was designed for high heat flux IGBT cooling. The geometrical parameters of the test section are as follows: outline dimension 229 mm × 124 mm × 30 mm; flow channels of 229 mm × 3 mm × 4 mm in total of 20. The experiments performed at atmospheric pressure and with inlet temperatures of 25–35°C, heat fluxes of 3.5–18.9 kW/m2. The influence of temperatures, heat fluxes on IGBT surface temperature and the cooling effect of the liquid cold plate have been investigated under a range of flow rates of 280–2300 kg/m2s. It was found that the heat transfer enhancement was very obvious using this kind of small sized channel for IGBT cooling, which was tens of times of the effect than air cooling or triple of the effect than that in normal sized channels. And the heat transfer enhancement increases with increasing heat fluxes and flow rates, while it decreases with increasing inlet temperatures. Most of the experimental results show good cooling effect as expected. However, it is dangerous for the cooling system under high heat fluxes when the system starts or stops suddenly, when the Respond Time (RT) is less than 5 seconds to cut off heated power. Also, the cooling performance is bad when the heat fluxes increased greatly, which is considered as abnormal situation in operating. The effect on IGBT surface temperature of heat flux is more obvious when the average Nusselt Number is smaller. For hydraulic characteristics observed, it was found that the flow friction increased with flow rates increasing, but the pressure drops of heated flow channels ahead were slightly larger than those back, especially under large flow rates conditions. That is because the temperatures of flow heated in channels ahead are lower than those back, which causes the fluid viscosity to be higher. At last, this paper suggested a series of method for enhancing heat transfer in flat plate heat sink, and also gave some ways to avoid heat transfer dangerous situations for IGBT cooling, which can provide a basis for thermodynamic and hydraulic calculation of flat plate heat sink design and lectotype.