In this paper, the thermal performance of chip scale packaged (CSP) LED was studied by numerical simulation and demonstrated by experiments. Heat dissipation characteristics of CSP LED modules which were packaged upon an aluminum substrate with a diameter of 25 mm and a thickness of 1 mm were computed by finite volume method, and the study showed that the chip junction temperature was closely related to chip power and distance between chips. To achieve technical requirement of junction temperature below 120 ℃, the distance and chip power both need considering. When the distance was 3.5 mm, only the single chip power of 0.5 W could meet the requirement. As the distance increased, the chip power could rise gradually. When the distance was 7.25 mm, chips whose power was less than 3 W could be applicable to the packaging. While the total input power was certain, thermal resistance of module decreased with the increased of packing density. When the packing density was 15.13%, module thermal resistance dropped to 2.26 K/W. Chip scale package is development direction of the next generation of LED packaging because of the lower thermal resistance.