Abstract Pyrolysis of defatted Euglena gracilis was investigated in this study to maximize energy recovery from algal biomass after lipid extraction. Prior to pyrolysis of defatted E. gracilis, the growth rate of E. gracilis was monitored to determine its potential as an initial carbonaceous feedstock for pyrolysis. This study revealed that the cell density of E. gracilis linearly increased for the first 5 days, during which the cell density reached 6.06 ± 0.82 g L−1, demonstrating that defatted E. gracilis is a promising feedstock for pyrolysis. To increase the thermal efficiency of defatted E. gracilis pyrolysis, CO2 was employed as a reactive gas medium. CO levels were increased by 45% following pyrolysis of defatted E. gracilis in a CO2 environment compared to in an N2 environment. Considering that CO is a highly combustible permanent gas, the use of CO2 in pyrolysis may result in the production of more fuel-range gaseous chemicals. Additionally, CO2 utilization increased the gaseous product yield compared to N2-pyrolysis for treating the defatted algal biomass while decreasing tar yield.