The difference between compressive and tensile flow stress of a material at a given strain termed as strength differential (S-D) effect, has been evaluated in case of four commercial steels via a series of heat treated conditions. The results have unequivocally established that the magnitude of S-D was maximum in the as quenched condition and tempering of the quenched structure led to a decrease in S-D. Spheroidised and/or annealed structures exhibited the lowest value of S-D. A linear relationship of S-D value with hardness and mean stress for each case has been established. Attempts have been made to explain the observed S-D effect in terms of models based on atomic mechanism and of continuum mechanics.