How the capillary structure of Pulsating heat pipe (PHP) affects its heat transfer performance is focused in this paper. Experiments are conducted on the miniature looped PHP, which hydraulic diameter is near 1mm and the cross-sections are triangle and square. The results demonstrate that the PHP performance of bottom heating mode is much better than that of top heating mode when the PHP tilt angle is changed. The PHP of square cross-section capillary can operate normally at even horizontal position. The PHP of triangle cross-section has a lower thermal resistance than that of square. The PHP of 1.5mm diameter has better performance than that of 1mm.