The pressure dependence of the colloidal phenomena of tranoparticles in hydrothermal water was investigated by both experiment and theory. Dynamic light scattering experiments show that diamond nanoparticles, which are highly stable in ambient water, easily aggregate in high-temperature and high-pressure water. Although the stability of nanoparticles in ambient pure water does not depend on pressure, it is interestingly found that at constant temperature particles aggregate faster in the hydrothermal regime when the pressure is higher. A theoretical interpretation is proposed to predict the stability of colloids in water as a function of temperature and pressure. Numerical analysis shows that the repulsive interparticle potential barrier, which stabilizes particles in the dispersion, decreases dramatically in high-temperature and high-pressure water. The decrease in the potential barrier arises from the temperature and the pressure dependencies of the dielectric constant (E) and the ion product (pK(w)) of water. Numerical analysis shows that the pressure dependence of e is negligible in the temperature range of 20-300 degrees C, whereas the pressure dependence of pK(w) is significant at temperatures of T > 150 degrees C. The theory reveals that the pressure dependence of the rate of size increment in the hydrothermal regime results from the pressure dependence of pK(w). An increase in pressure in the hydrothermal water enhances the ionization of water molecules which reduces the surface potential of the particles. This effect lowers the interparticle repulsive potential barrier, which accelerates aggregation of the particles.