Nowadays, fly ash is recycled to make more eco-friendly building materials and reduce landfill area of coal-fired power plant. However, the high amount of natural radionuclides contained in fly ash could potentially pose radiological risks to people living in buildings made from these materials. The results revealed that the 226Ra, 232Th and 40K activities for the commonly used building material were in the range from 10.1 to 254.9 Bq kg-1, from 16.6 to 176.9 Bq kg-1 and from 21.2 to 1240.3 Bq kg-1 and 569.1 Bq kg-1, respectively: High gamma activity concentration for fly ash is due to the origin of fly ash and coal enrichment process of coal-fired power plant, in contrast, sand and stone samples which contain high radon concentration. Additional fly ash in concrete can increase or decrease the radioactivity of building materials, in which its variation depends on the percentage of the fly ash and matrix composition of the mixture. Even though the average indoor annual effective doses were lower than the upper limit, the total annual effective doses were slightly higher than the recommended dose of 2.4 mSv-1 due to the exposure of natural sources by UNSCEAR. From this study, radiological effects of fly ash samples as concrete additive in Viet Nam could be evaluated for any practical circumstances before they are used.