The objective of this study is to examine the thermal co-pyrolysis process of waste plastics and used engine oil (UEO) as the effect of plastic material, temperature, and reactor diameter. High density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS) have been used as waste plastic samples. A 400-g sample, consisting of 250 g of waste plastic and 150 g of used engine oil, was introduced into the vertical tubular reactor. The experimental results show that the liquid yield produced during HDPE/UEO co-pyrolysis at a temperature of 450 °C is the lowest. At the same temperature, PP/UEO and PS/UEO co-pyrolysis generate a greater amount of liquid compared to HDPE/UEO. The results of the co-pyrolysis of waste plastic and UEO were subsequently compared to the results of the individual pyrolysis of waste plastic and UEO. The result shows that more liquid was produced by co-pyrolysis for all types of plastic at temperatures of 450 °C and 550 °C than by individual pyrolysis. The experiment’s results also show that co-pyrolyzing waste plastics and UEO produces more liquid at smaller diameters. The highest liquid yield of 69.85, 88.53, and 87.53% was obtained from the co-pyrolysis of HDPE/UEO, PP/UEO, and PS/UEO at a reactor diameter of 60 mm. In the case of co-pyrolysis of HDPE/UEO, the production of liquid increases with decreasing reactor diameter, increasing by 9.8% from 60 to 69.8%. Higher results were obtained from co-pyrolysis of PP/UEO and PS/UEO, with an increase of 27% for PP/UEO and 31.8% for PS/UEO.