In this paper, a new method for accurate characterization of MOSFETs' thermal properties is proposed. First, experimental verification of conventional AC conductance technique is conducted. We developed novel measurement techniques composed of AC conductance technique and four-probe gate resistance method. Using the new technique, it is clarified that MOSFETs' channel temperature under AC conductance measurement is not isothermal because of 'Static SHE', and that AC conductance technique is not suitable for complete thermal characterization. Next, we modified conventional pulsed IV method. It enables us to determine accurate channel thermal resistance and to obtain isothermal IV curve. The new pulsed IV method enables to measure accurate thermal properties of the devices.