In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (similar to0.8 wt-%C) specimens were oil quenched from 900degreesC to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746degreesC to produce dual phase steel structure in the core of specimens with martensite fractions of similar to25 or similar to50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950degreesC. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.