In this present study, the effect of filling ratio on the operation limit of HCLPHP (Horizontal Closed Loop Pulsating Heat Pipe) was investigated experimentally. The HCLPHP was made of stainless steel 316 tube (outer diameter = 1.6 mm, inner diameter = 1.0 mm) which meandered between the cooling and the heating sections with 30 turns (60 channels). R-134a was used as the working fluid and the filling ratio was set to be 40, 50 and 60%. The maximum heat transfer rate at the 40%-filling ratio was higher than other cases (50 and 60%). When the heat input exceeds a certain value which is the operation limit, the evaporator temperature rises as the liquid in the evaporator dries out. At the dryout condition, the increasing rate of the temperature was rapid without any fluctuations for the filling ratio of 40%, but becoming gradual with fluctuations for higher filling ratios.