A phase change material (PCM) emulsion was prepared by dispersing the PCM in water with the aid of emulsifiers, which had higher fluidity, higher thermal conductivity, and more flexible volume change than the solid liquid PCMs. However, a critical issue for their large-scale applications is phase instability due to aggregation and precipitation of the PCM droplets. This work was to develop stable PCM emulsions prepared with n-hex-adecane by manipulating the key factors and analyzing the emulsion properties including emulsifier combinations and process conditions, interfacial film properties, droplet size distribution, and rheology characteristics. The stability was further improved with the addition of SiO2 nano-particles. The SiO2 nano-particles also acted as an effective nucleating agent to reduce the degree of supercooling. The thermal performance for potential application in thermal energy storage systems was also examined. Eventually, a novel and highly stable PCM-in-water nano-emulsions with droplets on a scale of tens of nanometers and a transparent appearance was developed for potential application in active thermal energy storage systems.