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softwood, kraft pulping, mixing, strength properties, reinforcement In this study the effects of mechanical treatment of kraft pulp fibers, at different temperatures, on pulp and pulp strength properties were investigated. The beating demand increased while the tear index and zero-span tensile strength of treated pulps decreased as the treatment temperature was increased from 100oC to 170oC. The carbohydrate compositions measured as monosaccharides after acid hydrolysis of the pulps were the same. The small differences in fiber deformation of the bleached pulps could not explain the severe strength losses at 170oC. The fracture analysis, together with the results of a fractionation study of treated pulps, indicated that single fiber strength was lost. The results also indicated that the fiber damage resulting from mechanical treatment at high temperature under alkaline cooking conditions led not to a reduction in interfiber bonding ability