The formation of sigma phase in duplex stainless steels during isothermal aging is widely reported in the literature, but the influence of strain hardening prior to aging on the kinetics of formation of this phase was not a subject of systematic studies so far. The objective of this paper is to study the influence of cold work degree and aging time in aging at 650°C or 850°C of an UNS S31803 steel. Cold rolling was conducted, resulting in four sets of samples with 0%, 10%, 30% or 80% cross sectional area reduction. Those samples were submitted to isothermal aging between 10 minutes and 96 hours, followed by water quenching. Sigma phase quantification is possible after selective electrolytic etching with 10% KOH solution, and the volume fraction of ferrite is obtained with the aid of ferritscope. It is observed that increasing the strain hardening increases the possibility of heterogeneous nucleation of sigma phase within the ferrite grains, leading to increased rate of formation of sigma and consumption of ferrite with increasing aging time at both temperatures studied.