The co-gasification of a high density polyethylene (HDPE) blended with a biomass has been carried out in a two-stage gasification system which comprises an oxidative pyrolysis reactor and a thermal plasma reactor. The equivalence ratio was changed from 0.38 to 0.85 according to the variation of blend ratio between HDPE and biomass. The highest production yield was achieved to be 71.4 mol/h, when the equivalence ratio was 0.47. A large amount of hydrocarbons was produced from the oxidative pyrolysis reactor as decreasing equivalence ratio below 0.41, while the CO2 concentration significantly increased with a high equivalence ratio over 0.65. The production yield was improved by the thermal plasma reactor due to the conversion of hydrocarbons into syngas in a high temperature region of thermal plasma. At the equivalence ratio of 0.47, conversion selectivities of CO and H2 from hydrocarbons were calculated to be 74% and 44%, respectively. © 2016 Hydrogen Energy Publications LLC.