A study has been made of the cold deformation aging susceptibility of dual phase steel with 14 and 22% martensite by volume. It was found that yield strength (YS) and ultimate tensile strength (UTS) increased with increasing testing temperature up to 200°C and then decreased at 250°C which indicated that static strain aging is dominant up to 200°C. This aging is associated with free dislocations in the ferrite as a result of the austenite-martensite transformation, which become preferential sites for solute atom diffusion. At higher temperature (250°C) a softening effect is dominant due to tempering of martensite. It was also observed that at a given aging temperature YS, UTS and ΔY (increase in stress due to strain aging) increased with by volume of martensite. © 2008 Institute of Materials, Minerals and Mining.