The least-significant-bit (LSB) substitution which is commonly used in steganography would degrade the quality of host image significantly when embedding high-capacity information. To overcome this drawback, we present an optimal block mapping LSB method based on genetic algorithm, which is out of researchers? concern but fundamentally important. Additionally we discuss the rule to select the best block size. The main idea is to minimize the degradation of the stego image by finding a best mapping function between host and secret image blocks at global scope. The experimental result is promising. The advantage of our method over most commonly used LSB strategy is obvious in enhancing PSNR value while having a secluded statistic peculiarity.