This study is performed to investigate the effect of the size of reentrant cavities on the thermal performance of a micro pulsating heat pipe (MPHP). The flow and thermal characteristics of the MPHPs with each MPHP having a different size of reentrant cavities, along with a MPHP without reentrant cavities are experimentally obtained and compared. Silicon-based MPHPs with and without reentrant-type artificial cavities inside the channels are fabricated using MEMS techniques. The MPHPs have rectangular channels which are engraved on a silicon wafer with a hydraulic diameter of 6671AM. Ethanol is used as the working fluid. To allow for flow visualization, the etched micro-channels are covered with a transparent glass. Each MPHP with reentrant cavities has reentrant cavities of one size, which are either 10, 20, 30, or 40 mu m, respectively. Reentrant cavities in the MPHPs are shown to promote nucleation and early startup. Furthermore, the thermal resistance of the MPHP with reentrant cavities is decreased by up to 57%. As the size of the reentrant cavities increases, lower input power is required for startup, and the MPHP with the largest cavities (40 mu m) shows the earliest startup. On the contrary, as the size of the reentrant cavities decreases, a reduction of the thermal resistance of the MPHPs is maintained to higher input power, and the MPHP with the smallest cavities (10 gm) shows the lowest thermal resistance at high input power (> 12 W). Finally, a single MPHP incorporating various sizes of reentrant cavities (10, 20, 30, and 40 gm) is shown to exhibit extended operating range and enhanced thermal performance simultaneously.