Conventional fossil fuels are widely used as a source of energy for prime movers like IC Engines. The efficient prime movers like diesel engines are usually found applications in a wide array of areas, including industrial, agricultural and transport sectors. The demand for diesel and other petroleum-based fuels across the globe, especially in developing nations such as Ethiopia, has been rising steadily leading to concerns for long-term energy security. Due to a combination of these factors, various research initiatives aimed at developing suitable alternative sources of energy from waste plastic recycling continue to be accepted. To take its part, this thesis mainly concerns the extraction of liquid oil from waste PET and HDPE individually and from their mixture using a catalyzed pyrolysis process carried out in a batch reactor MS system. It searched for the maximum possible liquid oil yield in terms of presence of catalyst, temperature, residence time, feed consumption and waste plastic ratio. The presence of a natural zeolite catalyst is found to increase the liquid oil yield of the pyrolysis process. The conversion efficiency increases for pure PET (38.1 to 43.5 wt%), pure HDPE (72.8 to 84.6wt%) at 10% catalyst with a processing residence time of 45 minutes. The liquid conversion efficiency for 1:4 ratio of HDPE to PET is 65.3wt%, but it becomes 78.4wt% when the ratio is 4:1, processed at optimum temperature of 425 and 10% catalyst. The physical properties like density, calorific value, pour point and viscosity of the liquid oil extracted from waste plastic are finally tested using respective characterizing machines to justify its correlation with pure diesel fuel. The FTIR and TGA are used to investigate the functional group and weight loss, respectively, of the liquid oil extracted from waste plastic. Engine performance like brake powers ( ), brake torque (Tb) and brake specific fuel consumption (bsfc) were also evaluated. Numerically, PET, diesel fuel, HDPE, and mixed waste plastic fuels have an average brake torque of 2.455, 2.25, 2.315 and 2.36 Nm respectively. The Tb of PET waste plastic fuel is 0.205(8.37%), 0.14(6.05%) and 0.091Nm (3.86%) higher than pure diesel fuel, HDPE and mixed waste plastic fuel respectively. Diesel, HDPE, Mixed and PET fuels have average outputs of 1.79, 1.83, 1.94 and 2.06kW respectively. The fuel consumption of all fuels decreases with increasing the engine speed. When comparing the specific fuel consumptions to each other, the average fuel consumptions are 0.29, 0.285, 0.285 and 0.279 kg/kWh for pure diesel, HDPE, Mixed and PET, respectively. Keywords: Engine performance, alternative fuel, fuel properties, pyrolysis, waste plastic, catalyst