Data hiding is usually achieved by alternating some nonessential information in the host message. A more challenging problem is to hide data in a two -- color binary image. Hiding is difficult for the binary image since each of its black or white pixels requires only one bit representation. So that, changing a pixel can be easily detected. In this paper, we propose a new data hiding scheme using the parity of blocks. The original image is partitioned into m x n blocks. The new scheme ensures that for any bit that is modified in the host image, the bit must be adjacent to another bit that has the same value as the former?s new value. Thus, the existence of secret information in the host image is difficult to detect. The invisible effect will be achieved by sacrificing some data hiding space, but the new scheme still offers a good data hiding ratio. Specifically, for each m x n block of host image, we will hide one bit of secret data by changing either one bit or without changing any bits in the block.