To clarify the character of the relationship between the threshold stress and coherence, Al-Al₃Zr alloy with coherent particles was chosen in this work. High temperature creep test particle strengthened Al-Al₃Zr alloy was carried out by means of stress relaxation test at 573K, 623K and 673K. For the experiments, an Al-0.55wt%Zr alloy was melted in Ar gas atmosphere and then chilled to an ingot. The alloy was strengthened by coherent particles, β`(Al₃Zr) with Ll₂ structure. It was found that there exist the threshold stress during stress relaxation test and the threshold stress, σ_(th) results from the presence of β`(Al_Zr) particles. It was also obtained that the stress are approximately equal to Orowan stress, σ_(or) at 573K but 0.4σ_(or), at 673K. The temperature dependence of threshold stress might be due to the interaction between dislocations and particles. The ratio of σ_(th) and σ_(or) decreases gradually with increasing temperature. During local climb a considerable amount of length of dislocation increase lead to the threshold stress which depends on particle shape and is roughly estimated by σ_(th) =(0.4∼0.9)σ_(or). The activation energy for relaxation creep is found to be very close to that for pipe diffusion of pure Al. Observation of deformed specimens by transmission electron microscopy(TEM) suggests that dislocation and particles interact as a repulsive type.