Effect of double cross sectional ratio on performance characteristics of pulsating heat pipes is experimentally investigated. The cross sectional ratios in the evaporator and condenser sides are different from each other, and this novel gradually constricted geometry provides to boost flow circulation and to maintain a stable performance regardless of orientation. Tests are performed for different values of inclination angle (0 degrees, 30 degrees, 60 degrees and 90 degrees) and filling ratio (20, 40 and 60%). Also, results of originally structured closed loop flat plate pulsating heat pipe (novel design, ND) are compared with those of the conventional pulsating one with uniform cross section (conventional design, CD). Measurements are supported by high speed flow visualization. It is shown that the ND shows better thermal performance than the CD, and when appropriate filling ratio (40%) is provided, the ND presents orientation-independent running. A turn of the ND is divided into three different zones, and a pressurebalance equation is presented following a detailed flow analysis. Rapid bubble growth instability and annular flow pattern characterized by thin film evaporation are two important characteristics observed for the ND.