A new method has been presented by which the contents and elemental composition of biomass and plastics in the wastes is quantitatively determined. A new two-stage pyrolyzer was proposed to determine simply the contents of biomass and each plastic from the gas analysis during the pyrolysis of mixture under no interaction state. It was found that the contents of polyethylene, polypropylene, polystyrene, and biomass is determined from the ethane, propylene, C4 compounds, and CO2 yields, respectively. Next, a new method has been proposed to determine the elemental composition of each component by combining CHO diagram and the calorimetric value of the mixture. The estimated values were coincided with the real values within 10 % error. Thus, it was clarified the proposed method is valid for estimating the quality of waste materials to stably operate the treatment process of wastes such as combustion, pyrolysis, gasification etc.