Abstract: The RTM process specimens of glass fiber continuous mat/E51 epoxy resin with different void content were prepared by applying different injection pressure (0.1-0.35 MPa). The void content of the specimen was measured by ultrasonic method, metallography and density measurement/resin burnout method. The relationship between void content and resin injection pressure was investigated. Effect of void content on ultrasonic parameters and mechanical property of RTM composites was discussed. The results show that, the change of resin injection pressure has a significant effect on the void content. The injection pressure increases from 0.1 MPa to 0.35 MPa, and the void content of glass fiber continuous mat/E51 epoxy resin composites decreases from 9.95% to 3.73%. Ultrasonic characteristic parameters are nearly linearly increased with the increasing of void content, and especially the change of ultrasonic nonlinear characteristic parameters are more obvious. The change of ultrasonic parameters can evaluate void content of composite materials.