Sargassum tenerrimum has been used for the production of bio-oil by hydrothermal liquefaction in water subcritical conditions at a temperature range of 260-300 degrees C for reaction time 15 min. Maximum bio-oil (16.3 wt%) was obtained at 280 degrees C and maximum conversion was observed (75.8 wt%) at 300 degrees C. The solid residue yields decreased continuously from 61.2 wt% to 24.2 wt% as the temperature increased from 260 to 300 degrees C. The liquid products obtained upon hydrothermal liquefaction were characterized with the help of H-1 NMR, GC-MS and FTIR techniques and bio-residue using FT-IR and XRD. Analysis of bio-oil showed that their components and functional structure of bio-oils were greatly distinguished and consisted of many compounds including phenols, ketones, aldehydes, acid, esters, alcohols, nitrogen-containing compounds, and hydrocarbons. From the GC-MS analysis it has been seen that, organic acid content in the bio-oils were decreased with the increasing temperature from 260 to 300 degrees C. The band at 1083 cm(-1) only appeared in the absorption profile of S. tenerrimum feed, which could be C-O connected with hydroxyl groups and were dehydrated after liquefaction. All the biooils showed higher percentages of aliphatic protons in the spectral region from 0.5 to 3.0 ppm. The analysis of bio-residue obtained at various temperatures showed typical amorphous nature that indicated richness of carbon content in the residue.