Abstract: A non-unique corresponding relationship was verified between porosity P and ultrasonic attenuation coefficient α in carbon fibre reinforced polymer (CFRP). In order to explain the relationship from the point of ultrasonic scattering mechanisms, the CFRP models containing various size of voids (P=0.5%~3.5%) were established according to a kind of unidirectional CFRP laminate with thickness of 2 mm manufactured by autoclave moulding process. And the values of ultrasonic attenucation coefficient α were obtained by numerical calculation method. When the transverse dimension D of void is 56 μm and the normalized wavenumber kD=2π Dλ <1 (the wavelength λ ≈ 560 μm), α increases linearly and slowly with the increasing of P. And α shows logarithmic growth with the increasing of P when D=93 μm (kD ≈1). The simulation results show that, the ultrasonic attenuation may include two kinds of mechanism of Rayleigh scattering and Stochastic scattering depending on the normalized wavenumber when the ultrasound propagate in CFRP containing voids. The random and complex void morphology causes a non-unique corresponding relationship between porosity and ultrasonic attenuation coefficient.