Subcritical water technology gained much popularity being utilized in different applications due to its advantage of being a green, cost-effective, environmentally friendly technology with a higher quality of extracted product and considerably short reaction time as compared with traditional solvent extraction methods. The biomass residue from agriculture, forest wood production, and the food and feed processing industry, could be an important feedstock for producing high-quality products at lower production cost and high-efficiency. Valuable biological active compounds, such as antioxidants an important group of health-protecting compounds, could be extracted under subcritical water conditions from agricultural residual biomass such as fruits, plants, and vegetable. Using subcritical water technology to produce nanocrystals showed that the yield increased linearly with increasing the pressure, therefore, the production process requires a reactor that supports high pressures but results in a greener method and lower effluent generation.