The optical and thermal properties of the CSP LEDs were investigated as function of the phosphor film thickness and width. Eight different types of phosphor-converted white CSP LEDs were prepared by using mixed multicolor phosphors. It was shown that with increasing phosphor film thickness, the luminous flux and the radiation angle increased. On the other hand, as the CSP area increased, the radiation angle decreased. This behavior was attributed to the fact that the blue light of the LED did not excite the whole area of the phosphor. At high current, the junction temperature increased with increasing phosphor film thickness and the thickest phosphor sample showed the highest junction temperature. The results demonstrate a simple method of optimizing the phosphor layer thickness and width for the fabrication of high-performance white LEDs for a white light source.