Abstract: Glass fiber reinforced composites with different amounts of voids were prepared by applying different curing pressures (0.1~0.6 MPa). Ultrasonic C-scan and ablation density method were adopted to measure the attenuation coefficient and the average porosity. The distribution, shape and size of the voids were characterized by metallographic microscopic analysis. The effects of void content on the tensile, bending and interlaminar shear properties and the attenuation coefficient were investigated and discussed, and the critical value of void content and attenuation coefficient were also obtained. The results show that, with the decreasing of curing pressure, the voids content of specimen increases from 0.976% to 5.268%, the tensile strength, flexural strength and interlaminar shear strength decrease, the ultrasonic attenuation coefficient increases from 1.460 dB穖m^-1 to 2.150 dB穖m^-1, and the critical attenuation coefficient is about 1.5 dB穖m^-1.