The required pressure of the mixing chamber of an internal mixing twin fluid atomizer, related with the mass flow rate and the geometry of atomizers are discussed from the basis of fluid dynamics of two phase fluids. The equation of jet velocity ejecting through an orifice port is deduced using compressibility of two phase fluid For the acoustic velocity within a two phase fluid, Tangren's equation is used. The equations were calculated numerically and compared with that of the experimental data of mixing chamber pressure, P1 and mass flow rate ratio, μ, of air and liquid, where μ=Wa/W1 and two phase mass flow rate is, Wm2=W1 (1+μ). As the results, it was found that the critical pressure Pc at the pressure of transient to the acoustic velocity is Pc=060P1. Above P1>163.3 kPa, i.e. P2=Pc>98.07 kPa the jet velocity passing through the nozzle throat is controlled by the acoustic velocity. By the judgment of this critical pressure, the jet velocity expressed by the orifice equation, was deduced The tendency of the calculated values from equations was agreed adequately with that of the measured data and correcting coefficient related with throat area ratio of nozzles, (D2/D1)2 was obtained.