Pyrolysis, a thermal cracking process in inert environment, may be used to produce bio-oil from biomass and plastic waste thus accommodating the use of renewable energy. Abundant amount of biomass wastes in Indonesia are not utilised and plastic wastes are not well processed for clean environment. The aim of present work was to evaluate effect of mass ratio of plastic material to biomass in the feed blend of corn cobs and high density polyethylene (HDPE) of co-pyrolysis on bio-oil yield and chemical composition of bio-oil products. The heating rate of the co-pyrolysis was kept low and residence time was in the order of seconds to accommodate high yield of oil originating from plastic pyrolysis. Corn cobs have high cellulose and hemicellulose content (84%) which is potential to produce bio-oil. The pyrolysis was conducted in a laboratory-scale using a fixed bed reactor with final temperature of 500 °C, heating rate 5 °C/min, flow rate N2 750 mL/min, total weight of biomass and plastic material of 20 g, and hold time after peak temperature of 30 min. Mass ratio of plastics to biomass in the feed blend was varied 0:100, 25:75, 50:50, 75:25 and 100:0. It was found that by increasing HDPE content up to 100% in the feed blend above, the yields of bio-oil decreased, i.e. 28.05, 21.55, 14.55, 9.5, and 6.3 wt%, respectively. Furthermore, for the same variation of the mass ratio, yields of the non-oxygenate mixture of paraffins, olefins and cycloalkanes contained in bio-oil increased, i.e. 0, 28.35, 40.75, 47.17, and 67.05 wt%, respectively. Analysis of the synergetic effect indicates that this effect did work effectively both in bio-oil and non-oxygenate yields due to improper conductive heat transfer from N2 gas to melting plastics, which contained biomass particles. By increasing composition of HDPE in the feed blend, viscosity and pH of bio-oil changed approaching to those of commercial diesel oil.