Abstract: A MF Laser Scatter Sizer and an experimental system designed by the authors were employed to measure the sprays produced by coaxial injectors of LO_x/LH₂ liquid rocket engines at back pressure in the vessel.Experimental results show that injectors work as both airblast and pressure nozzles;keeping the air/liquid mass flux ratio (ALR) constant,the atomization performance is improved when the air pressure drop (△P_g) is increased; and enhanced back pressure results in bad atomization of the liquid.Experimental correlations between spray mean size SMDand its main influential parameters have been obtained by fitting experimental data.Several injectors with different configurations have been investigated and the results obtained from the experiments provide valuable reference for injector design.