The objective of this study was to study the effect of contact force of ultrasonic probe on the ultrasonic attenuation and power spectrum of radish. The relationship between attenuation(y: neper/m) and contact force(x: kgw) can be expressed as y=a+bLnx, and the regression coefficients a and b were influenced by porosity and volume respectively. The relationship between power spectrum and contact force also has the same regression form, the coefficient a was influenced by porosity. The attenuation and power spectrum respectively decreased and increased with the increase of contact force. The correlation coefficient between physical properties of radish and attenuation was influenced by contact force.