(1) We have calculated the binding energy of vacancy pairs in NaCl and the other five alkali halide crystals. The calculations is improved at several points compared with that by Reitz and Gammel. The calculated values of the binding energy are considerably smaller thane2/kaand differ from the result by Reitz and Gammel; for instance, 0.44 ev in NaCl, 0.37 ev in NaBr, and 0.58 ev in KCl crystals.(2) As the first step of the systematic investigations of dislocation problems, we have calculated the core energy and the Peierls‐Nabarro critical yield stress of the edge dislocation for slip in the (110) plane in NaCl type crystal [the Burgers vector isa(110)]. If we write the energy of the dislocation at the center of a cylinder with radiusRasKln (R/R0),R0is about 1 A in most of the alkali halide crystals. The results of the calculation concerning the Peierls‐Nabarro yield stress permit us to understand the well‐known fact that LiF has much larger critical yield stress compared with the other alkali halide crystals.