The　Viterbi　algorithm　is　a　method　based　on　the　Maximum-Likelihood　estimation;　it　detects　the　one　that　is　most　likely　to　have　generated　from　the　received　sequence.　This　paper　researched　the　Viterbi　decoding　algorithm　of　ERP-OFDM　physical　layer　pattern　in　IEEE802.11g　system,　analyzed　the　algorithm　performance　based　on　the　distance　metrics　using　correlative　value,　uniting　the　character　of　the　Gray　code　constellation　mapping.　The　simulation　result　indicated　that　adopting　the　correlative　value　as　the　distance　metrics,　the　new　algorithm　has　a　good　decoding　performance,　and　is　easier　to　realize　by　hardware,　which　compared　with　the　usual　Viterbi　decoding.