Abstract: Based on the theoretical analysis for the porosity，density and longitudinal velocity，the acoustic impedance for the carbon fiber unidirectional reinforced composite was measured using the ultrasonic immersion technique with the bottom echo reflection and a 5 MHz flat probe. The metallographic microscope was used to determine the actual porosity. An empirical equation was built for testing porosity with acoustic impedance based on the parameters calibration. The experiments for samples with porosity from 0.03% to 2.21% show that the porosity obtained from the acoustic impedance model basically accords with that from the metallographic method. According to this method，the composite porosity could be determined conveniently without measuring the material velocity and density. Influenced little by the morphology of the pores，the determination of the acoustic impedance has feasibility on nondestructive testing for composite porosity.