In the present study, combined influence of filling ratio, geometry, inclination angle and heating power on heat transfer characteristics of the pulsating heat pipes (PHPs) are investigated accompanying analysis of complex thermophysical mechanism. Three different flat-plate type closed loop PHPs covering uniform or non-uniform channel-pairs are investigated at different angles (0 degrees - 90 degrees) and filling ratios (50% and 80%). It is concluded that phase change phenomena and bubble dynamics play key roles on thermal characteristics. In addition to unbalanced capillary force, evaporation momentum force is an important factor to improve operational performance of non-uniform PHPs in the lack of gravity support. Non-uniform design decreases the influence of gravity on results. At horizontal position for medium filling ratio (FR = 50%), uniform PHP (PHP1) collapses. Compared to angled conditions, maximum heat input of PHP1 drops from 65 W to 45 W (nearly 30% decrease). At horizontal orientation, non-uniform PHPs maintain operation, and perform better performance than PHP1. For a relatively high filling ratio (FR = 80%), effect of inclination angle is more obvious, especially for uniform PHP. In the existence of liquid-excess (at FR = 80%), a new type operation mode is defined as column type bulk condensation flow.