Using constrictive air and transformer oil as working substances an experimental study is made on pressure characteristic in the mixing chamber of gas-liquid two phase internal -mixing-oil-nozzle in the paper. The result of the research shows that, the main operation factors on the pressure characteristic in the mixing chamber are the oil pressure (p1), the gas pressure (p2), and the difference between these two pressures (p1 - p2) according to the given oil nozzles. In traditional design and calculation for internal-mixing nozzle, the mixing-chamber pressure is regarded as the product of atomizing medium pressure (p2) and critical pressure ratio(b ) of this atomizing medium in some references. But from the experiment it can be seen that the pressure in the mixing-chamber is not equal to p2b, due to that the critical pressure ratio cannot be reached in practical operation. Therefore, the pressure data obtained by this method should be corrected to meet the case of error existence. The relations about pressure character in the mixing chamber of the gas-liquid two-phase internal-mixing nozzle gained in the paper can provide reference for theoretical study and design on the gas-liquid two-phase discharge characteristics of this kind of nozzles.