•Permeability and effective pore radius measured for mesh-groove wick and mesh wicks.•Heat pipes tests conducted for composite mesh-groove wick and two-layer mesh wick.•Heat pipes tests compared for both wicks under different inclinations.•Maximum heat loads of mesh-groove wick 4 times higher than same-thickness mesh wick.•Qmaxs calculated using measured wick properties agree well with measurements. After the thermal performance of a flat-plate heat pipe with sintered composite copper mesh-groove wick has been shown to be superior in our previous study, the capillary properties, namely the permeability (K) and effective pore radius (reff), are characterized in this work using the capillary rate-of-rise method. The composite mesh-groove wick consists of a layer of 200-mesh copper screen sintered over parallel semi-elliptic grooves with a width of 0.18 mm and a depth of 0.075 mm. While the measured reff for the composite mesh-groove wick agrees well with the prediction by a literature formula, the measured K is about a half of the theoretical prediction available in the literature, which does not account for mesh layer. A new formula of K calculation for the composite mesh-groove wick is proposed with the mesh layer accounted for. The new theoretical K agrees excellently with our measurement. Also investigated is a sintered two-layer 200-mesh (2 × 200 mesh) wick for comparison. Our measured K and reff of it also agree well with the experimental values in the literature. The experimental K/reff value for the composite mesh-groove wick is about 4 times as large as that of the 2 × 200 mesh wick. Then, visualization experiments for flat-plate heat pipes adopting either of these two wicks are conducted under different inclinations. The maximum heat loads (Qmaxs) measured for the composite mesh-groove wick are about 4 times as large as those for the 2 × 200 mesh wick, in agreement with the ratios of the K/reff values of these two wicks. In addition, the measured data of K and reff are used to calculate the Qmaxs of the heat pipes with either wick under various orientations. The theoretical Qmaxs compare well with the experimental measurements.