The defect imaging contrast on the section topographs for the crystal of potassium acid phthalate (KAP) has been studied, using the synchrotron radiation. The experimental results show that the diffraction contrast due to defects in the crystal is considerably increased with the wavelength increasing in a certain range The dynamical theory of X-ray diffraction for homogeneously deformed crystals with average values of the strain gradient parameter βis taken to interpret the experimental results, for which the factor β teff plays an important role in the defect imaging contrast formation We propose that it is possible to choose an appropriate wavelength of synchrotron radiation which could give out higher diffraction contrast for defects in crystals.