Provided is a biodegradable thermoplastic resin composition including a cellulose derivative and surface-treated natural fiber. More particularly, in preparation of the biodegradable thermoplastic resin composition including the cellulose derivative and the natural fiber, a surface of the natural fiber is treated with alkali and/or sized, thereby increasing a density of the natural fiber, and an interfacial adhesion between the cellulose derivative and the natural fiber to improve a mechanical strength of the biodegradable thermoplastic resin composition. Furthermore, as the cheap natural fiber is used as a reinforcer, the production cost can be reduced.