Disclosed is a one coat painting composition comprising a base including a polyester resin in an amount of about 15 to 45 wt %, a first acrylic resin in an amount of about 15 to 25 wt %, and a second acrylic resin in an amount of about 35 to 65 wt % and a coating method thereof. The composition makes it possible to maintain and improve a high gloss finish of a high gloss injection molded substrate through a single primary coating on the substrate and satisfies environment resistant properties of exterior parts, such as adhesiveness, weather resistance, and scratch resistance, and, thus, is widely suitable for use on exterior plastic parts of automobiles.