Abstract: Through theoretical calculation, numerical simulation and physical experiments, we studied the variation of amplitudes of the diffracted wave at location of the top point of the crack with different width of vertical crack which is in the underground medium. The results show that: (1) Width of crack less than the diameter of the first Fresnel zone is not able to be measured directly from the image of ground penetrating radar. But with the increase of the width of crack, the amplitude of the diffracted wave is continuously enhanced. Especially, when the width of crack is less than half the diameter of the first Fresnel zone, the amplitude of the diffracted wave versus the width of crack has the approximate linear relationship. (2) When the width of crack is approximately equal to the diameter of first Fresnel zone, the amplitude reaches the maximum. (3) After that the amplitude decrease gradually and until it be stable to a constant value as width of crack is far more than the first Fresnel zone and the width crack is able to be measured directly from theimage of ground penetrating radar. The research results can be used in recognizing the underground crack and quantitatively estimating the width of the crack.