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Abstract. Hydrothermal liquefaction (or HTL) of microalgae biomass is a promising conversion technology in which wet biomass is treated under high temperature (280-350 ^oC) and pressure (700-3000 psi), and the associated water in the wet biomass forms the reaction medium. A significant fraction of the microalgae cell is converted into biocrude, which separates from the aqueous layer by gravity and/or chemical/mechanical methods. Since the HTL process uses the whole biomass, increasing lipid content during microalgae cultivation is not essential. With an overall goal to utilize highly productive algal strains into a liquid hydrocarbon fuel/ fuel precursor, the current research investigates HTL conversion of a marine algae feedstock (Tetraselmis) and a freshwater algae (Spirulina) in the presence of co-solvents. HTL was performed using a 1.2 L Parr reactor at 275-350 ^oC for 30 min in the presence or absence of a co-solvent (ethylene glycol and isopropyl alcohol). The results showed that Tetraselmis algae preferred severe conditions and biocrude yield increased with temperature increasing from 16% -25%. Addition of 10% of co-solvent promoted biocrude oil conversion. The yield at high temperature with co-solvent didn‘t change significantly, while the yield with co-solvents at low temperature was much higher (29%). Spirulina produced 40-45% biocrude yield under all HTL conditions. This study also reports the characteristics of the obtained biocrude.