ASTC is one of the standard texture formats supported in OpenGL ES 3.2 and Vulkan 1.0 (and later versions), and it has been increasingly used on mobile platforms (Android and iOS). ASTC’s most important feature is the block size configuration, thereby providing a trade-off between compression quality and rates. With the higher number of textures, however, it is difficult to manually determine the optimal block sizes of each texture. To solve the problem, we present a new approach based on PSNR values to automatically determine the ASTC block size. A brute-force approach, which compresses a texture on all block sizes and compares the PSNR values of the compressed textures, can increase the compression time by up to 14 times. In contrast, our three-step approach minimizes the compression-time overhead. According to our experiments on a texture set including 64 various textures, our method determined the block sizes from 4x4 to 12x12 and reduced the size of compressed files by 68%.