The experimental design was used to determine the effects of the reaction temperature (180–260 °C), the reaction time (0–2 h) and the sawdust concentration (9.1–25 wt%) on the biomass conversion, the product yields and product properties during the hydrothermal liquefaction of pine sawdust. The determined conversion and the aqueous product (AP) yield were in the range of 23.1–57.2 wt% and 14.6–43.4 wt%, respectively. The pH of the slurry product was 2.54–3.44, while the higher heating values (HHVs) of the attained solid residue (SR) and the heavy oil (HO) were in the range of 21.3–28.3 MJ/kg and 21.6–29.4 MJ/kg, respectively. Comprehensive mass and carbon balance model predictions were built. They can be used for predicting the appropriate reaction condition to obtain the desired HTL product. The results showed that all experimental variables had a statistically significant effect on the conversion and the liquid product yields. An increase in the reaction temperature improved the conversion, the liquid product yields (except for the carbon yield of AP), the acidity of the slurry product and HHVs of SR and HO. The reduction of the sawdust concentration led to an increase in the conversion and the liquid product yields, while a very short reaction time favored the liquid product formation. The lignocellulosic degradation products in AP were qualitatively analyzed by means of the GC-MS analysis. The kind of furan, cyclic and one oxygen-containing species produced in AP depends on the reaction temperature and the reaction time.