Hydrothermal liquefaction of Auxenochlorella pyrenoidosa (AuP) and Arthrospira platensis (ArP) at 350 degrees C for 1 h produced algal biocrudes (BCs), BC(AuP) and BC(ArP), with yields of 41.82 and 36.60 wt.Wo, respectively. These two algal BCs were cut into five distillate fractions (DFs) of 25-100 degrees C (DFO), 101-200 degrees C (DF1), 201-300 degrees C (DF2), 301-400 degrees C (DF3), and >= 401 degrees C (DR) using atmospheric distillation under N-2 atmosphere. The total yields of DF1, DF2, and DF3 from either BC(AuP) or BC(ArP) are at least 60 wt.%. All the DFs, from either AuP or ArP, showed different yields and elemental and molecular compositions. Next, the DF1, DF2, and DF3 DFs were each blended with used engine oil (UEO) at a mass ratio of 1:1 and treated at 400 degrees C for 4 h with an additional 0.1 kg(Pt/C)/kg(feed) under 6 MPa H-2. The presence of UEO could dilute the DF, avoid solvent extraction of the product oil, favor desulfurization of the upgraded oil, and be directly recovered as a major part of the product oil. Catalytic hydrotreatment of the DF and UEO blends led to a higher upgraded oil yield (> 79 wt.%) and lower coke (< 12 wt.%) and gas (< 9 wt.%) yields compared with those from the BCs alone under the same process conditions, and higher upgraded oil yields were achieved when using the DFs with high boiling point ranges. The upgraded oil had a lower total acid number and oxygen, nitrogen, and sulfur contents than those of the BC. The sulfur contents of the upgraded oil produced from the DF and UEO blends were much lower than those from the BCs alone, and the lowest sulfur content of 12 ppm (w/v) was achieved. The high abundance of unsaturated hydrocarbons and nitrogen- and oxygen-containing compounds in the BC were replaced by a high abundance of hydrocarbons and benzene derivatives in the upgraded oil. The heating value of the upgraded oil ((similar to)48 MJ/kg) was higher than that of the BC. The main gas-phase products were H-2, CH4, C2H6, and C3H8 Overall, many of the properties of the upgraded oils obtained from the catalytic hydrotreatment of the DF and UEO blends were similar to those of hydrocarbon fuels derived from fossil fuel resources.