The issue of high air temperature inside busy long vehicular tunnels becomes more and more noticeable and serious in countries with a hot summer time. This paper studies on the feasibility and applicability of the water mist cooling system in Hsuehshan tunnel. The water mist diameter is the key point for proper water mist cooling design in tunnel. The droplet diameter was calculated according to mathematical model and it should not affect drivers’ visibility. A Computational Fluid Dynamics (CFD) program was adopted to investigate the key parameters in water mist cooling, such as relative humidity, tunnel air speed, water flow, and water temperature. Based on the results, the design parameters were finalized. A full scale model was setup to investigate the performance of the water mist cooling technology to be adopted in Hsuehshan tunnel. From the results of the full scale test, it was concluded that air temperature could be effectively decreased without reducing visibility.