In order to study mechanisms of high temperature deformation of NiAI intermetallic compound with a B2−type crystal structure， interna正stresses（σ∂and effective stresses（σ，）have been estimated by stress−relaxation tests conducted in the temperature region from 1223 to 1373 K． Two methods，孟θ．， Kikuchl’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 NiA1． The internal stresses and effective stresses estimated from the two methods are in relatively good agreement with each other． The ratio of internal stressσ6 to flow stressσs，（i．e．，σノσ，）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 102〜103が， indicating that effective stress in NiAI is related to dragging stress of jogged screw dislocations． Thus mobile dislocations in NiAI at high temperatures move viscously as a result of the dragging stress acting on logged screw dislocations and not as a result of the lattice friction associated with the Peierls stress．