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Martensitic stainless steels are commercially significant materials owing to the remarkable properties they offer because of their martensitic structure during application. The paper aimed to investigate the effects of tempering temperature on mechanical properties and microstructure. Determination of carbide morphology was also included in the study to substantiate the results. This study used AISI 420 samples of 10 mm thickness, hardened at 1050oC and then subjected to tempering treatment at temperatures: 150oC, 250oC, 350oC, 450oC, 550oC and 650oC. Each sample was oil quenched after a soaking time of 25 minutes. Optical Microscopy and Scanning Electron Microscopy was conducted to examine the changes in the microstructure and the morphology of carbides. The results have shown that at various temperatures during tempering there is subsequent increase in toughness where hardness decreases and vice versa. Matrix of martensite contains some carbide precipitates. Due to Temper embrittlement occurring between 350°C-450oC, toughness has been decreased as a result of secondary hardening between the temperatures of 450°C-550°C, hardness has been increased drastically.