Biodegradable polymers, which can be degraded into non-toxic elements, metabolized, absorbed and excreted, are utilized as drug carriers and depots in drug delivery systems. They are frequently used in injectable formulations such as soluble nanoparticles, microspheres, and gelling system. In this chapter, we focus on polylactide and the copolymers as popular biodegradable polymers, and the developments of drug delivery system by hybridization of polylactides with some hydrophilic polymers are introduced.