The invention is injecting carbon dioxide into hydrothermal liquefaction (HTL) in order to increase biocrude product yield and/or decrease byproduct yield. 1) Injection of carbon dioxide into a hydrothermal liquefaction process including, but not limited to In the feeding section And/or the preheating section And/or the reactor section And/or the product handling and collection section 2) The carbon dioxide is optionally obtained from the waste product gas from an HTL process This invention improves product yield, reduces byproduct yield, and may enhance operation of HTL when carbon dioxide is injected into one or more unit operations into an HTL process. As a source of CO2 is available from the HTL process byproduct gas, it can be easily collected, recompressed, and injected into the HTL system at various locations. As the CO2 is captured from the HTL process, this also enables simple recovery/recycle of the injected carbon dioxide as a normal function of HTL. This recovery may also be used to capture carbon dioxide in order to reduce atmospheric emission.