In this study, we theoretically and experimentally investigated the thermal performance of a thin flat vapor chamber (TFVC) with a multiscale wick to understand the effect of the additional capillary microstructures on the cooling performance. To fabricate the multiscale wick with the additional capillary microstructures, micro-sized particles of SAC305 (Sn3.0%Ag0.5%Cu) and Sn58Bi were mixed in ethanol, including indium flux. The suspensions were sequentially sprayed onto a conventional screen mesh wick and spontaneously sintered at 150 ℃. The rate-of-rise method was used to measure the capillary performance of the multiscale wick experimentally. When the results were compared with that of the conventional screen mesh wick, the effective pore radius and permeability of the multiscale wick were 45.4 and 29.5% lower than those of the conventional screen mesh wick, respectively. The capillary performance of the multiscale wick enhanced up to 30% higher than that of the conventional screen mesh wick. With the measurement data of the multiscale wick, the thermal performance of TFVCs with multiscale wicks was theoretically obtained. Based on the theoretical results, it is shown that the maximum heat transfer rate of the TFVC using the multiscale wick increased by up to 82.8% compared with the conventional screen mesh wick.