How pulsating heat pipes (PHPs) reach their operational limit has not yet been fully understood. This study aims to provide a complete picture of the termination mechanism of the self-oscillation of vapor and liquid. Experimental studies on a 10-turn PHP with HFC-134a were conducted and the filling ratio (FR) was from 20% to 80%. The thermo-fluid behavior in the PHP was investigated by temperature measurements with a high-resolution and high-speed infrared camera and estimation of fluid-to-wall heat flux distributions by solving inverse heat conduction problems. The results suggested that the PHP, increasing heat load, reached the operational limit due to different mechanisms depending on the filling ratio: at a high FR (80%), the liquid volume ratio increased with the increase of the operating temperature, resulting in the compressed liquid phase. At a low FR (20%), when a large amount of heat was applied, the fluid in the evaporator dried out and became a superheated vapor. The PHP with an optimum FR (50%) transferred the maximum heat under the same evaporator temperature, as the fluid in the PHP was able to keep the saturated two-phase state until the evaporator temperature exceeded the critical temperature.