In the present investigation, the hardness of low carbon microduplex steels consisting of lower bainite and martensite was evaluated as a function of the volume percent of lower bainite. In addition, a series of the microstructural examination was carried out in detail through optical microscopy. SEM and TEM in order to elucidate the effect of the microduplex microstructure of these steels on the hardness. The present investigation was conducted as one of the preliminary studies to find the new steel microstructure for bolt which can exhibit the enhanced strength and delayed fracture resistance simultaneously without major compositional modification. The microstructural characteristics of the present microduplex low carbon steels were manifested by the presence of the needle type lower bainite partitioning the prior austenite grains. Hardness measurement, of these steels showed that, without tempering, there existed an optimum volume fraction of lower bainite showing maximum hardness. The hardness was higher than that of martensite single phase. However, after tempering, a definite hardness peak was not observed but hardness of tempered microduplex steels was somewhat higher than that of tempered martensite single phase up to 70% of lower bainite volume fraction. The analysis of the microstructure and phase transformation characteristics revealed that the strengthening of lower bainite - martensite microduplex structure was attributed to (a) the refinement of martensite lath due to the partition of the prior austenite grains by lower bainite and (b) increase of martensite hardness due to carbon redistribution during austenite - lower bainite transformation.