Abstract: Slow co-pyrolysis and fast co-pyrolysis of cellulose (CE) with high density polyethylene (HDPE) were investigated in a tube furnace reactor. The effects of heating rate and mixing ratio on the co-pyrolysis of CE and HDPE were investigated. The results show that slow co-pyrolysis of CE and HDPE can boost liquid yields while lowering gas and char yields. When the ratio of CE to HDPE is 1∶3, the slow co-pyrolysis interaction is strongest. At this ratio, the liquid yield was 95.4%. Compared with the calculated yield, the liquid yield is increased by 9.8%. Rapid co-pyrolysis reduce the liquid yield, but increase the gas yield. When the ratio of CE to HDPE is 3∶1, the rapid co-pyrolysis interaction is strongest. The liquid and gas yields were 49.3% and 42.8%, respectively. Compared with the calculated yields, the liquid yield is reduced by 14.5% but the gas yield is increased by 14.1%. These results verified that the synergistic effect existed during co-pyrolysis of CE and HDPE under the experimental conditions. Co-pyrolysis of CE and HDPE favors reducing the oxygen content of the liquid product and improving the quality of the liquid product.