Abstract: A heat pipe is one of the heat transfer elements to effectively remove the heat from the high-temperature surface. A flat-plate micro heat pipe (FPMHP) has been developed to explore the heat transfer characteristics in previous studies. However, it is very necessary to further enhance the FPMHP heat transfer performance, particularly in antigravity. An effective way can be to increase the capillary force for the higher heat-transfer function against gravity because the capillary structure can pose a significant influence on the thermal performance during operation. In this study, the copper foam with different pore diameters was employed to place inside each micro heat pipe for the higher capillary force. The wire mesh-copper foam and micro-grooved wick were combined to form a composite wick, in order to promote the backflow of condensate for the more nucleation points at boiling. The copper foam was located between the micro-fins on the upper and lower surface, where there was no direct contact with the upper and lower surface. The main functions of copper foam were: i) To enhance the evaporation/boiling heat transfer; ii) To reduce the pressure drop of the backflow of condensate; iii) To enhance the condensation heat transfer. Three groups were also set to clarify the influence of the pore diameters of copper foam on the FPMHP thermal performance, including the anti-gravity (inclination angle of less than 0° was the angle between the FPMHP and horizontal plane), micro-gravity (inclination angle was 0°), and gravity (inclination angle was more than 0°). The results were as follows. The better heat-transfer performance of FPMHP was achieved at the micro-gravity, where the thermal resistance of FPMHP with the composite wick was lower than that with the single grooved wick. There was also an enhancement effect of composite wick on the FPMHP heat-transfer performance. But, a seriously deteriorated effect was found under anti-gravity operation, when the inclination angle was less than -10°. Furthermore, the temperature difference between the evaporation and condensation section was also analyzed to further explore the FPMHP performance. Specifically, better thermal performance was gained, as the temperature difference was reduced significantly. Therefore, the best FPMHP performance was achieved when the inclination angle was 0°, due mainly to the smaller temperature difference. The minimum thermal resistances of the FPMHP with different pore diameters were 0.13, 0.17, and 0.13 K/W, respectively, which were far lower than those with the grooved wick. Compared with the FPMHP without copper foam, the increase rates of thermal conductivity of FPMHP with pore diameters of 0.2, 0.5 and 0.8 mm were 3.57, 2.43, and 3.54, respectively, indicating the better thermal performance of the FPMHP using a composite wick. A comparison was also made on the current heat pipe, where the minimum thermal resistance of the FPMHP was significantly lower than those. Therefore, the copper foam can be expected to improve the heat transfer performance of the FPMHP with the composite wick. As such, the lower thermal resistance can rapidly remove the heat generated by the heating element. The findings can also provide strong theoretical support to enhance the heat transfer of heat pipes for the application in the field of thermal control.