Tensile tests were performed using both water-quenched and tempered Fe-0.2 mass％ C-0.5 mass％ Si-2.5 mass％ Mn-(0-0.82) mass％ V steels so as to elucidate the effects of V addition on strengthening mechanisms in tempered martensite steel. Microstructure and precipitates were observed, and dislocation density was analyzed. Because vanadium carbide (VC) is plate-shaped and has a Baker-Nutting orientation relationship with the ferrite matrix, the size & shape of VC on the slip plane were considered in evaluating precipitation strength for tempered 0.82 mass％ V steel. The increase in the yield strength of tempered steel by adding 0.82 mass％ V can be mainly explained by both VC precipitation strengthening and dislocation strengthening due to high dislocation density. This study also suggests that precipitation strength could be overestimated on the assumption that precipitation strengthening is caused only by the Orowan mechanism.