This study aims to reduce radon gas emanations in the indoor environment by incorporating kenaf and oil palm nanocellulose that act as nano-fillers into building materials. Fabrication of composite brick was carried out according to the MS and ASTM standards. In this research, 40, 80, 120, 160 and 200 ml of nanocellulose were used to replace the usage of sand, stone and cement materials, respectively. Kenaf and oil palm nanocellulose were utilised to reduce the internal and surface porosity as well as to replace the radon resources (stone), which indirectly reduced radon gas emanation. Radon gas emanated from each composite brick was measured within 10 consecutive days in an airtight prototype Perspex room using Radon Monitor Sentinel 1030. A compression test was also carried out to investigate the physical strength of the fabricated composite bricks. The results showed that 40 ml of kenaf and oil palm nanocellulose was the optimum amount in reducing the radon concentration, where the radon readings were 1.4 and 0.93 pCi per l, respectively. Meanwhile, the brick with no nanocellulose exhibited the highest radon reading of 3.77 pCi per l. Moreover, the Young modulus for the composite brick of both kenaf and oil palm nanocellulose was 28.92 and 27.8 N per mm2 compared to the control brick, which was 27 N per mm2. The results proved that radon gas emanations were reduced by 62.86% for kenaf and 75.3% for oil palm by incorporating the organic nanocellulose, which has high potential towards a healthy indoor environment.