According　to　Maxwell＇s　equal　area　law　we　study　the　＂real＂　phase　diagram　of　the　charged　AdS　black　hole　and　the　Kerr-AdS　black　hole.　In　the　extended　phase　space　constructed　by　treating　the　cosmological　constant　as　pressure,　the　two　kinds　of　AdS　black　hole　display　themselves　like　van　der　Waals　system.　Under　the　critical　temperature　T　(c)　,　there　exists　a　regime　where　the　condition　of　stable　equilibrium　will　be　violated.　We　employ　the　equal　area　law　to　find　an　isobar　which　is　the　real　two　phase　coexistence　line.　As　a　byproduct　we　find　a　simple　method　to　derive　the　critical　value　of　the　thermodynamic　quantities.