Among the indoor air pollutants, radon gas which is a natural radioactive material is known as the second leading lung cancer cause. 80∼90% of infiltrated radon gas is caused by underground soil and foundation under houses, schools and buildings. Also, radon concentration can be worse in case of buildings where there are plenty of cracks on the walls or the floor and where there shows lack of ventilation rate. While in other developed countries and World Health Organisation(WHO) devise active counterplans, Korean government still has not shown a big step. This study was about producing nano-size carbon colloid and applying it to filters. After the application, radon mitigation efficiency assessment was performed. Meanwhile, the efficiency assessment was compared to one of a normal medium filter and a carbon filter. For this study, the radon chamber designed with a strict condition was used. In case of normal filter and carbon filter, they showed about 22% of mitigation efficiency under high radon concentration. However, the filter treated with nano-sized carbon colloid mitigated about 37% of radon under the same condition.