This study argues that CO2 can be used to modify the composition of bio-oil generated from hydro thermal liquefaction of two different microalgae such as Microcystis aeruginosa (M. aeruginosa) and Nannochloropsis oceanica (N. oceanica). Prior to hydrothermal liquefaction, the lipid content of microalgae was quantified as similar to 2 wt% of M. aeruginosa and similar to 14.5 wt% of N. oceanica. In addition, the thermal degradation of M. aeruginosa and N. oceanica was characterized to evaluate the influence of CO2 via quantifying major pyrolytic gases (i.e., H-2, CH4, and CO). Hydrothermal liquefaction of these two microalgae was performed using a high-pressure batch reactor in N-2 and CO2 atmospheres. The composition of chemical species containing N and 0 in bio-oil was significantly decreased under hydrothermal liquefaction in CO2, which meant that the quality of microalgae-derived bio-oil was improved. In addition, the use of CO2 allowed the production of more petro-diesel like bio-oil. This study suggests a practical means for utilizing CO2 for the production of high-quality bio-oil from toxic and economic microalgal biomass. (C) 2017 Elsevier Ltd. All rights reserved.