Environmental concerns due to fossil fuel usage has turned the research interest towards biomass and bioenergy field. Renewable biomass such as microalgae provides numerous advantages as they can grow in wastewater; sequester carbon dioxide, economical and eco-friendly. In this study, effect of pretreatment of microalgae (Scenedesmus obliquus) biomass using post-hydrothermal liquefaction wastewater (PHWW) for bio-oil production through hydrothermal liquefaction at a temperature of 300 °C was studied. Results showed liquefaction of pre-treated biomass yielded 48.53% bio-oil whereas 28.35% was resulted from biomass without pretreatment. The analysis of higher heating value of bio-oil showed that pretreated biomass oil has 36.19 MJ.Kg-1 against non-pretreated biomass oil, which has 28.88 MJ.Kg-1. Bio-oil (pretreated biomass) analysis revealed that 60% of compounds are in diesel and gasoline range with 58.09% of energy recovery. Bio-oil was rich in hydrocarbons of C7-C21 range with less oxygenated compounds. Carbon balance showed that an increase of 13% of carbon was sequestered in solid residue obtained from pretreated biomass and about 146% of increase also obtained in bio-oil.