An experimental investigation has been made of characteristics of a rectangular, annular single-phase natural circulation loop with the inner tube filled with a solid-liquid phase change material (PCM) or air under cyclic pulsating heat load. A rectangular, annular loop of 150 cm in height and 75 cm in width was constructed with an annular gap of 0.6 cm, within which water was filled. The inner tube of the annular loop was filled with a PCM (n-Eicosene) or air. Under the cyclic pulsating heat load, the temperature within the water-filled annular loop with PCM- or air-filled inner tube were all found to evolve into a steady periodic variation for the range of parameters considered. The water temperature and/or its fluctuating amplitude along the heater or cooler sections of the loop with the PCM-filled inner tube were found to be markedly lower than those measured in the loop with the air-filled inner tube under the identical conditions. On the other hands, along the insulated sections of the loop a somewhat minute difference in temporal variations of the water temperatures exists between the loops with PCM- and air-filled inner tube. In addition, at the outer wall along the cooler section, a time-periodic variation of temperature was detected in synchronizing with the pulsating heat load. Parametric effects of varying amplitude and period of the pulsating heat input, as well as of varying the inlet temperature of the water-cooled jacket were investigated.