A preliminary investigation of the biocarbon fraction in carbon dioxide emissions of power plants using both fossil and biobased fuels is presented. Calculation of the biocarbon fraction is based on radiocarbon content measured in power plant flue gases. Samples were collected directly from the chimneys into plastic sampling bags. The 14C content in CO2 was measured by accelerator mass spectrometry (AMS). Flue gases from power plants that use natural gas, coal, wood chips, bark, plywood residue, sludge from the pulp factory, peat, and recovered fuel were measured. Among the selected plants, there was one that used only fossil fuel and one that used only biofuel; the other investigated plants burned mixtures of fuels. The results show that 14C measurement provides the possibility to determinate the ratio of bio and fossil fuel burned in power plants.