GaN-based power device is expected to be widely used in power converter system, owing to its superior characteristics of high-speed and high-efficiency, it is important that the dead-time related loss of GaN-based HEMT devices have negative impact on improving system efficiency in application of buck converter. In order to accurately assess the influence of the dead-time related loss on converter efficiency, analytical model combined with simulation results for dead-time related loss of GaN-based devices upon power converter system has been discussed and optimized in this letter. The different operating principle of buck converter system is discussed in Si-MOSFET and GaN HEMT and the mechanism of dead-time related loss is analyzed, respectively. For a converter of input voltage 12 V to output voltage 12 V, load current 20 A operating at a switching frequency of 700 kHz, the optimal efficiency is 92% under the conditions of Td is 20 ns, Te is 0 ns.