Ultrasonic phase spectrum of reflection coefficient has been used to nondestructively characterize the density, porosity, and microcracks of plasma sprayed Cr2O3 coatings irradiated by high-intensity pulsed ion beam (HIPIB). The ultrasonic measurement was investigated using immersion focusing pulse echo method with a 25 MHz transducer and the phase spectrum of reflection coefficient has been experimentally obtained. The relationship among velocity, density, attenuation coefficient and the phase spectrum was analyzed based on an acoustic transmission model in a multi-layered structure. The velocity relates to the harmonic frequency of the phase spectrum, and the density and attenuation coefficient play roles on the amplitude of the maximum and the shape of the spectrum. The effects of the three parameters (velocity, density and attenuation coefficient) on the amplitude of the maximum of the phase are similar. Some critical value exists which change the relationship between the individual parameter and the maximum of the phase spectrum. When one of the three parameters exceeds the corresponded critical value, the amplitude of the maximum decreases in the form of exp( ) n y = A Bx . For the as-sprayed and irradiated coatings with the thickness of 50 0m, the velocity calculated from the phase spectrum is 2522 and 2772 m/s, harmonic frequency corresponding to 12.61 and 13.86 MHz, respectively. These ultrasonic determinations demonstrated that the coating becomes more densified after irradiation by HIPIB, in agreement with SEM observations. Ultrasonic phase spectrum itself is a good indicator to reveal differences between density and microstructures of coating prepared with various conditions as well as to follow their evolution after HIPIB irradiation.