Abstract: When the metal solidifies the porosity may form in the castings as a result of evolvenment of gases from metal, Technological methods of adopting outer pressure upon solidfying castings(Such as solidifacation under gas pressure, antipressure casting, liquid forging etc).can effectively reduce the microporosity in castings. However, the outer pressure can reduce not only the evolved, gas porosity, but discrete porosity caused by shrinkage of metal. Because of this, up to the present work, there hadn't been any method for measurement of the effect of outer pressure on eliminating only evolved gas porosity in castings. In this article, after theoritical analysis, a method for mesuring the effect of outer pressure on eliminating evolved gas porosity in castings has been described, a mathematical equation for determination of this effect has been given too. At the same time such measurement with the AL-4. 5 %Cu alloy has been carried out by means special equipments and instruments. It was found, that the enlargement of outer gas pressure from O to 4kg/cm2 can eliminate 68% evolved gas porosity caused by increment of hydrogen content in metal from 0.187cm3/100g to 0.365cm3/100g.