Abstract: On the basis of ultrasonic nondestructive testing, using Sylva test-Duo ultrasonic detector to test the healthy wood and internal different sizes defects wood, achieved the quantitative relationship between ultrasonic propagation velocity and hole diameters. The analysis results showed that the radial velocity of ultrasonic spreading in the healthy wood was range of 1 423 -1 627 m/s2; the wood internal hole defects could cause the significantly decrease of the radial velocity of ultrasonic spreading. The relationship between the ultrasonic propagation velocity and the size of the defect diameters were fitted to curves, and defect size was significantly negatively correlated with radial velocity, the high fitting degree of >08. Exponential function model and a linear function model were adopted to analyze the relationship between the ultrasonic propagation velocity and the size of the defect diameter，and found that the ideal mathematical model of different types defects was different.