Abstract: Microstructure characteristics of dual-phase steels consisting of ferrite plus martensite obtained by using various heat treatments were observed carefully, and effect of the characteristics on tensile properties of dual-phase steels were analysed. Results show that art. Important feature of dual-phase steels is "phase-hardening" occurred in martensite and it phase-hardening" occurred in ferrite,, The strength et !itese steels depends root only on the volume fraction of martensite, but also on the aspect ratio of martensite islands. More. Deviation was found when the strength value of these steels calculated by the law of mixture originated in composite materials reiforced by long fibres was compared to, that obtained by experiment.