Co-pollution　of　surface　O-3　and　PM2.5　has　become　the　most　predominant　type　of　air　pollutions　in　Beijing-Tianjin-Hebei　region　in　the　hot　season　since　2017,　particularly　in　May-July.　Analysis　based　on　observational　data　showed　that　co-pollution　was　always　accompanied　by　high　temperature,　moderate　relative　humidity,　extremely　high　SO2,　and　higher　NO2.　We　also　found　that　the　meteorology　and　precursor　dependence　of　O-3　was　similar　between　co-pollution　and　O-3-　single　pollution.　While　PM2.5　in　co-pollution　was　more　related　to　temperature,　relative　humidity,　and　precursors,　that　in　PM2.5-singe　pollution　were　more　related　to　small　winds.　These　results　indicate　that　co-pollution　seemed　to　be　more　affected　by　atmospheric　chemistry.　According　to　the　PM2.5　components,　secondary　inorganic　aerosols　(SIA)　composed　44.3-48.7%　of　PM2.5　in　co-pollution,　while　those　accounting　for　42.1-46.5%　and　41.2　+1.3%,　respectively,　in　O-3-　and　PM2.5-single　pollution,　which　further　confirmed　the　relatively　stronger　atmospheric　chemistry　processes　in　co-pollution.　And　the　high　proportion　of　SIA　in　co-pollution　was　mainly　attributed　to　SO42-,　which　was　observed　to　rapidly　boom　in　non-refractory　submicron　aerosol　(NR-PM1)　on　the　condition　of　high　level　of　O-3　at　daytime.　Additionally,　we　further　explored　the　interactions　of　O-3　and　PM2.5　in　co-pollution.　It　was　found　that　most　(similar　to　61.9%)　co-pollution　episodes　were　initiated　by　high　O-3　at　daytime;　while　for　other　episodes,　high　PM2.5　firstly　occurred　under　the　more　stable　meteorological　conditions,　and　then　accumulation　of　precursors　further　induced　high　O-3.　A　higher　SIA　concentration　was　observed　in　O-3-initiated　co-pollution,　indicating　that　the　atmospheric　oxidation　in　co-pollution　caused　by　chemical　processes　was　stronger　than　that　by　physical　processes,　which　was　further　approved　by　the　higher　values　of　SOR　and　NOR　in　O-3-initiated　co-pollution.　This　observational　study　revealed　that　controlling　O-3　and　precursor　SO2　is　the　key　to　abating　co-pollution　in　the　hot　season.