Characterization of the flow properties of plastics melts is one of the key factors for the successful operation of the plastics pyrolysis plant. Experiments for the viscosity measurement of plastics melt are carried out on the mixed plastics used for RDF, recycled HDPE, recycled LDPE, recycled PP, and mixed plastics being made according to the generation ratio of waste plastics. Experiments are performed to find out the following relationships : viscosity with the mixture ratio of plastics and waste oil, viscosity with temperature, and viscosity with the time elapsed at constant temperature. Experimental results show that viscosity is increased in log scale as the mixture ratio of plastics increase. When temperature is raised far enough above the melting point, viscosity dependence on temperature can be expressed as . Viscosity of the plastics melt decrease as time elapsed at constant temperature. Analytical result of the TGA and GPC confirms that decrease in viscosity is caused by the decrease of the molecular weight owing to the pyrolysis of the plastics melt in the viscinity of the melting vessel wall.