•Large pores and small pores reduced flow resistance and sustained capillary force.•Driving force was composed by capillary force, phase-change force, gravity force.•Formation of vapor phase in compensation chamber caused temperature pulsation.•Two operation modes were concluded and minimum thermal resistance was 0.382 °C/W.•Gravity assistance trebled the thermal performance than horizontal condition. Loop heat pipe, acting as one of the applications of porous media, has been extensively researched for its reliability and robustness. In order to enhance the heat transfer capacity of the loop heat pipe and adapt it to long-distance thermal control system, high-performance capillary wick is urgently required. In this paper, a comprehensive study about the biporous wick fabrication and operating mechanism is introduced. Contrasting to the conventional monoporous wick, the coaction of large and small pores could reduce the flow resistance and sustain sufficient capillary force. The driving force for loop circulation was comprised of capillary force, phase-change driving force, and gravity force. Furthermore, a flat-plate loop heat pipe with long heat transfer distance was constructed to test the performance limit of the biporous wick. Test results under horizontal condition indicated that the loop transferred the maximum heat load of 110 W (heat flux of 6.6 W/cm2) and the minimum thermal resistance was 0.382 °C/W. Besides, the intermittent formation of vapor phase inside compensation chamber would cause the temperature pulsation during low heat load range. While increasing the heat load, the subcooled liquid became sufficient and the pulsation was restrained. By introducing gravity effect at a tilt angle of 4.6°, the working performance was improved for the wick could be quickly infiltrated by the subcooled fluid even under long heat transfer distance. The maximum heat load was 330 W (heat flux of 19.9 W/cm2) and the minimum thermal resistance was 0.161 °C/W at a heat load of 240 W.