We propose an interchannel parallel coding scheme in the wavelength domain -- the WDM coding system. The system differs from usual serial coding systems and provides many advantages. First, data channels are completely unaltered in the coding process, rendering it very suitable for practical lightwave systems with standard bit rate. Secondly, parallel encoding/decoding are simpler than those of serial coding systems, being easier to be implemented in high-speed optical systems. Thirdly, comparing to serial coding, WDM coding is able to reduce heavily the number of encoding/decoding pairs: an example Hamming coded WDM system reduces the number from 12 X 11 equals 132 to 1 at the line rate of STS-12. Fourthly, WDM coding system could offer infinite coding gain in dispersion-limited lightwave systems. Finally, WDM coding system could correct single channel burst error.