In the present study, a series of experiments are conducted to investigate the thermal performance of a copper made pulsating heat pipe consisting of uniform flow passages with the cross section of 2 mm x 2 mm. Test conditions cover two orientations (0 degrees and 90 degrees) and six different filling ratios (10%, 25%, 40%, 55%, 70% and 85%). The working fluid used in the experiments is methanol. Heat inputs are applied by 7 W intervals up to an upper safe temperature limit (mean evaporator temperature of nearly 110 degrees C). In addition to temperature measurements and relevant thermal resistance values, the flow behavior is analyzed via high speed video images. It is concluded that at vertical bottom heating mode (90 degrees), the filling ratio plays a key role in the results, and thus, obvious differences occur in thermal performance depending on the filling ratio. As a general trend, at vertical position, thermal resistance increases with increasing filling ratio for a given heat input value. As an exception, the lowest filling ratio (10%) significantly disobeys this generalization. Thus, the worst thermal performances are obtained for the lowest and topmost filling ratio values (10% and 85%). Nearly for every filling ratio, the system can operate at vertical position, while the system cannot start up and/or properly operate at horizontal position (0 degrees). When the heat pipe is placed horizontally, the effect of filling ratio on the thermal behavior significantly diminishes. As an overall evaluation (including flow patterns and evaporator temperature), the optimum thermal performance is obtained for the filling ratio of 40% in existing conditions.