Duckweed was treated over a temperature range 250-374 degrees C, a retention time of 5-90 min, and a catalyst loading of 0-10 wt% by a high-pressure reactor. Operating temperature, retention time, and catalyst loading are all found to affect oil yield. The lower limit of reaction temperature for the production of heavy oil was found to be 260 degrees C. The highest oil yields were obtained at 30% on an organic basis under the following conditions: reaction temperature of 340 degrees C, retention time of 60 min, and operation without catalyst. The average heating value of the bio-oil product was estimated at 34 MJ/kg.