In this paper, we studied the block LZSS algorithm and investigated the relationship between the compression ratio of block LZSS and the value of index or length. We found that as the block size increases, the compression ratio becomes better. We also found that the bit of length has little effect on the compression performance, and the bit of index has a significant effect on the compression ratio. We showed that the more the bit of index is set, the bigger optimal block size is obtained.