The soot formation rate at high pressures has been studied with a premixed propane-oxygen-inert gas combustion in a constant-volume vessel. The soot formation rate was measured by the laser extinction technique at the conditions of: 0.4 to 5.0 MPa for pressure, 1600 to 1900 K for temperature and 2.0 to 2.2 for the equivalence ratio. The burnt gas temperature was measured by the two-color method. It was found that the soot formation rate can be expressed by empirical pseudo first-order kinetics for surface growth even at high pressures in the present study, and that the rate constant of soot formation is only dependent on the temperature.