A novel method of LED chip on board (COB) package based on plastic radiator without substrate is proposed. The thermal properties of tradition COB package and this new structure are simulated by using finite element analysis software Ansys. The simulation results show that, by mounting the LED chips on the plastic radiator with thermal conductivity of 20 W/(m·K), the method of LED COB package can provide a much lower temperature than the metal substrate COB package, and is similar to the ceramic substrate COB package. By further simulations, it is found that the thermal resistance is the minimum when the thickness of the plastic radiator is 3.9 mm. Moreover, with the thermal conductivity and the convection coefficient increasing, the thermal resistance reduces in different degrees. Because of the easy preparation process and colorful luster of the plastic, this new method of LED COB package has a broad application prospect.