The present invention disclosed a medium-chain-length poly (3-hydroxyalkanoates) (MCL-PHA) for use as natural based eco-friendly plasticizer for poly (vinyl chloride) (PVC). In the present invention, MCL-PHA was biosynthesized by pseudomonas putida pga1 via fermentation using oleic acid and palm kernel oil as renewable carbon source. MCL-PHA is a source of green natural polymers which can be used to improve material properties of PVC. Compare with the conventional petrochemical based plasticizer, MCL-PHA would not be hazardous to human health and the environment due to the non-toxic, biodegradable and biocompatible properties.