The microscopic defects in AlPO4 crystals have been studied by means of X-Ray diffraction topography. It was found that the major defects in these crystals are growth layers, precipitaties and dislocations. The density of dislocations is maximum near the crystal surface, and is less in the center of the crystal. The dislocation originates in thermal stress and the latticdistortion induced by the precipitaties or growth layers. The Burgers vector of most of dislocations is of the type b=(a+c) 〈1123〉, and that of the others is b=a2110. The relationship between growth conditions and crystal defects are analysed. We think that for improving the perfection of crystals, it is important to control temperature fluctuation during crystal growth, and especially the cooling rate after the growth process.