Abstract: Quenching followed by tempering is one of the most popular heat treating processes for steels. But it is usually observed that the strength and ductility of steels vary according to temperature-time parameter of tempering in opposite ways. It should be noted that these conventional strength and ductility values are obtained from unidirectional tension tests with smooth specimens. The actual load carrying capacity or service life of a machine part, however, does not follow closely with either strength or ductility of the material uniquely, but is determined by a suitable combination of strength and ductility according to service conditions.This paper presents results of investigations on the effect of tempering upon some of the parameters which more nearly characterize the load carrying capacity and service life of a few constructional steels quenched to fully martensitic structure. These parameters include:Resistance against fracture under monotonic loading-ultimate tensile strength of notched specimens with various stress concentrations right up to an actual crack and notch sensitivity factor, fracture toughness and impact toughness;Resistance against fracture under cyclic loading-endurance limit of smooth and notched specimen and endurance life due to over loading, endurance life due to repeated impact. Stress cycles for crack initiation and rate of crack propagation.Experimental results obtained show that nearly all of these parameters exhibit some peak, valley or saturation values at certain tempering temperatures, each corresponding to a suitable balance between strength and ductility of the steel prescribed conditions, This might throw some light on the selection of tempering temperature.Thin film electron-micrographs taken by a high voltage TEM reveal the structural changes which occur at different tempering temperatures. An, analysis of these changes provides some evidences for the above observations.