The present invention relates to biodegradable plastic composition comprising rice powder and/or corn powder, which can be characterized in comprising 100 parts by weight of polyolefin matrix resin; 5 to 400 parts by weight of grain powder selected from the group consisting of rice powder, corn powder and mixture thereof. The biodegradable plastic composition according to the present invention can be manufactured in various forms such as injection molding product, sheet molding and blow molding product, which have excellent physical properties and product stability. The efficiency of waste disposable of the product manufactured with the composition can be remarkably improved since the rive powder or the corn powder contained in the composition can be degraded by microbes in the nature after a certain period. Therefore, the problems of soil, air, and sea pollution caused by burial or incineration of the wastes of conventional plastic molding product can be minimized.