In　wireless　LANs　(WLANs),　the　Medium　Access　Control　(MAC)　protocol　is　the　main　element　that　determines　the　efficiency　of　the　wireless　channel.　In　this　paper,　we　focus　on　analyzing　IEEE　802.11　MAC　protocol　in　different　network　scenarios　systematically.　In　order　to　analyze　the　performance　of　IEEE　802.11　MAC　protocol,　a　performance　model　of　IEEE　802.11　protocol　was　built　using　Parallel　Object-Oriented　Specification　Language　(POOSL).　The　model　is　validated　from　two　aspects　of　function　behavior　and　theoretical　analysis.　The　performance　metrics　such　as　valid　throughput,　average　delay　and　probability　of　collision　were　analyzed　in　ad　hoc　network　and　infrastructure　network　with　access　point　(AP).　Through　simulation　of　the　model,　the　influence　of　the　parameters　such　as　minimum　contention　window　(CWmin)　and　number　of　active　stations　on　the　performance　metrics　was　given.　The　low-cost　CPU　execution　speed　of　AP　is　also　confirmed　by　analyzing　the　utilization　of　CPU.　The　methods　of　performance　modeling　and　analysis　in　this　paper　can　be　applied　to　study　the　performance　of　other　wireless　communication　protocols　such　as　802.15,　802.16,　802.20,　and　beyond.　©　2009　IEEE.