The ferroelastic domain walls in NdP5O14 crystals have been studied by means of X-ray fopography. It has been found that the ferroelastic domain walls appear to be black or white contrast in X-ray fopograph in the range of μt=0.6—7. The feature of the diffraction contrast of the ferroelastic domain walls is summarized as follows: (C2 - C1 = )△C//g contrast of the ferroelastic domain wall is strong △C⊥g contrast of the ferroelastic domain wall is invisible △C·g > 0 contrast of the ferroelastic domain wall appears to be black △C·g < 0 contrast of the ferroelastic domain wall appears to be white The diffraction contrast of the ferroelastic domain wall has been explained qualitatively by Penning-Polder's theory. According to the feature of the diffraction contrast, the structural characteristics of the ferroelastic domain wall has been discussed.