In order to study mechanisms of high temperature deformation of NiAl intermetallic compound with a B2-type crystal structure, internal stresses (σ_i) and effective stresses (σ_e) have been estimated by stress-relaxation tests conducted in the temperature region from 1223 to 1373 K. Two methods, i.e., Kikuchi’s method and Li’s method, are used to analyze the results of the relaxation behavior. The mobile dislocation densities remain constant during the relaxation tests in the temperature range tested so that Kikuchi’s method and Li’s method is relevant for analyzing the relaxation behavior of NiAl. The internal stresses and effective stresses estimated from the two methods are in relatively good agreement with each other. The ratio of internal stress σ_i to flow stress σ_s, (i.e., σ_i/σ_s) indicates that the flow stress includes the effective stress and that the pure metal type and alloy type high temperature deformation mechanisms coexist in high temperature deformation. Activation volume of the mobile dislocation is 10²∼10³b³, indicating that effective stress in NiAl is related to dragging stress of jogged screw dislocations. Thus mobile dislocations in NiAl at high temperatures move viscously as a result of the dragging stress acting on jogged screw dislocations and not as a result of the lattice friction associated with the Peierls stress.