Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass such as microalgae into bio-crude oil. Four main products, gas, bio-crude oil, an aqueous phase, and solid residue, are generated through HTL. In this study, various HTL conditions were investigated to enhance the phosphorus content in the aqueous phase as well as the yield of bio-crude oil. Tetraselmis sp. was used as the microalgae feedstock, and the product yields according to catalyst type were explored. The phosphate ion (PO43-) content in the aqueous phase was significantly enhanced when acetic acid was added because of the effect of pH. In addition, it was found that both the bio-crude oil yield and the ammonium ion content could be increased by recycling the post-HTL aqueous phase, while the phosphate content was not.