A　three-dimensional　mathematical　model　is　proposed　to　calculate　the　natural　ventilation　properties　for　an　isolated　room　with　different　opening　configurations.　The　Reynolds　averaged　Navier-Stokes　(RANS)　models　is　adopted　and　the　standard　k-　epsilon　turbulence　model　is　selected　to　study　the　wind-driven　cross　natural　ventilation.　Flow　around　a　building,　internal　ventilation　in　building,　pressure　field　are　described　and　circulating　flow　phenomenon　is　captured.　The　calculated　pressure　coefficients　on　the　mid-plane　of　building　are　compared　with　those　of　experimental　data.　The　results　show　that　the　steady　flow　pressure　coefficient　has　the　same　trend　as　the　experimental　results.　It　is　also　found　that　the　plane　of　ventilated　zone　seems　to　have　larger　at　exit　for　a　larger　exit　and　more　air　momentum　transport　would　be　carried　by　convection　with　the　increase　of　exit　area.　The　model　can　be　used　for　assigning　comfortable　locations　for　human　inside　the　room.