Microbial bioelastomers prepared by the autoxidation of the unsaturated medium-long chain length co-poly-3-hydroxyalkanoate's (mlcl-PHAs) based on soybean oily acids (Sy) have been reported. Pseudomonas oleovorans were grown on a series of the mixture of octanoic acid (OA) and Sy with the weight ratio of 20:80, 28:72 and 50:50 in order to obtain unsaturated mlcl-copolyesters coded PHO-Sy-2080, PHO-Sy-2872, and PHO-Sy-5050, respectively. The microorganism was also grown on the mixture of Sy and 10-undecenoic acid (UA) with the weight ratio of 50:50 to obtain unsaturated copolyester coded PHU-Sy-5050. The PHAs obtained were characterized by (1)H NMR and GC-MS techniques. Double bond contents of the unsaturated PHAs obtained were varying between 0.8 to 20 mol %. Autoxidation of the unsaturated copolyesters were carried out on exposure to air at room temperature in order to obtain new biomaterials whose mechanical strength was improved. Autoxidation kinetics, shelf life, mechanical and thermal properties of these biomaterials were evaluated.