Two-phase heat transfer devices based on capillary action in micro-scale porous structures such as loop heat pipes (LHPs) are promising heat transport technologies. This is because LHPs have a higher heat transfer capacity and a longer heat transfer distance than conventional heat pipes. Additionally, LHPs do not require electrical power to operate. As a result of its capillary forces, LHPs can operate in zero or anti-gravity conditions, making LHPs applicable to a variety of thermal applications, such as cooling or heat utilization in automobiles, electronic devices, and spacecraft. The thermal performance of LHPs is governed by the thermos-fluid behavior of microscale porous structures called wicks. Therefore, understanding the behavior of wicks is essential to develop higher-performing LHPs. In this study, microscale visible and infrared imaging was used to observe and model the thermal-fluid behavior of porous material surfaces. An observation system that can observe thermal fluid properties in micro-scale micropore structures in the infrared and visible regions has been developed for visualization.. Infrared observations reveal the relationship between the temperature field, maximum heat flux, and evaporative heat transfer. Visible observations show vapor-liquid interfaces, thin film evaporation, and nucleate boiling. The possibility of improving the heat transfer characteristics by improving the wettability of the heating surface and increasing the three-phase contact area around evaporating region was also verified. The capability of the heat transfer enhancement by increasing wettability and the length of the triple phase contact area on the heating surface were also evaluated. An optimal porous structure for LHP was proposed based on new findings. In LHP development, an LHP analysis model based on basic research knowledge was constructed. Using the constructed LHP analytical models, LHPs of various types (high heat flux, ultrathin, and large capacity) were designed, fabricated, and evaluated. For high heat flux type LHP, It was demonstrated that by controlling the pore size and vapor groove width, it is possible to achieve a maximum heat transport capacity of 200W and a maximum heat flux of 42.5 W/cm². For ultrathin LHPs, a 0.3 mm thick LHP for mobile devices has been developed, and a heat transfer capacity of 10 W or more was demonstrated. In the largescale LHP, the largest LHP is being developed with a heat transfer capacity of 6.2 kW in a single LHP for the purpose of thermal management in automobiles and waste heat utilization in factories.