In this study the thickness dependence of blue phases temperature range was evaluated at chiral nematic LC mixture (LC / chiral dopant) using the wedge cell. The temperature range of blue phase at the 0.5 mu m thick part is more than 10 K, while those at the 2 mu m and the 4 mu m thick part is less than 3K on cooling process. However, on heating process, the temperature range of blue phase is not nearly changed with changing of thickness of chiral nematic LC.