Window　opening　behavior　has　significant　influences　on　indoor　environment　and　energy　consumption　in　residential　buildings.　As　a　response　to　indoor　environment,　the　control　mechanism　(window　and　interior　door　open/closed)　should　be　studied　first　by　comprehensive　understanding　of　the　variation　of　indoor　environmental　conditions.　For　this　reason,　a　field　measurement　of　environmental　conditions　was　carried　out　in　five　representative　apartments　in　Beijing　from　April　to　May　in　2010.　By　comparison　analyses　of　the　field　measurement　results,　major　findings　are　as　follows:　(1)　the　concentration　of　room　CO(2)　can　be　the　best　predictor　of　occupant　behavior,　also　window　opening　behavior,　in　residential　buildings;　(2)　the　variation　in　indoor　air　quality　mainly　results　from　large　variation　in　window　opening　behavior;　(3)　apart　from　indoor　and　outdoor　thermal　environment,　indoor　air　quality　as　well　as　occupants＇　presence　at　room　have　also　considerable　effect　on　window　opening　behavior;　(4)　while　defining　window　opening　behavior　for　one　room　in　residential　building,　it　is　necessary　to　take　the　window　adjustment　of　its　connecting　room　and　the　adjustment　of　the　connecting　door　into　consideration　due　to　air　diffusion　between　the　two　connecting　rooms.