The chemical characteristics of oil feedstocks greatly affect the physical properties of biodiesel. Bacteria, because of their very high growth-rate and their easy culture, can be used for making oil-feedstocks to produce biodiesel. In this way genetic transformation was recently applied to microorganisms to improve oil quality and hence biodiesel quality. In this work, Escherichia coli was genetically transformed by heterologous expression of Arabidopsis thaliana stearoyl-acyl carrier protein desaturase3 (S-ACP-DES3): the cDNA coding for S-ACP-DES3 from Arabidopsis thaliana was cloned into the pET-15b vector and expressed in the E. coli BL21 (DE3) strain. The S-ACP-DES3 protein obtained was expressed in a soluble form after induction with IPTG and visualized by SDS-PAGE analysis. The recombinant Escherichia coli fatty acid profile showed an optimal unsaturated vs. monosaturated and saturated FAs combination. These results can be used as a starting point to try to modify oleaginous bacteria to get both high oil productivity and optimal oil composition.