<正> The characteristics of surface O3 on clear days at Waliguan Observatory,Lin'an regionalstation and Longfengshan regional station in China were analyzed in this paper.The three stationsbelong to Global Atmospheric Watch（GAW）of WMO.There was obvious daily variation on cleardays at Lin'an.with maximum（42.9 ppb）and minimum（20.3 ppb）of daily range appearing inspring and summer.respectively.The daily variation was more regular at Lonfengshan than atLin'an.The maximum（about 27 ppb）appeared in autumn at Longfengshan.There was noobvious daily variation and also daily range was smaller in other seasons except weaker dailyvariation in summer at Waliguan.But the surface O3 concentration（SOC）in summer was higherthan that in winter at Waliguan.The SOC on clear days of summer at Waliguan was over 20 ppbhigher than at Longfengshan and Lin'an.The global radiation and NOx concentration were themain factors which control the SOC on clear days at Longfengshan and Lin'an.and playedimportant role in different seasons and areas.The transportation of air flow around the area ofQinghai-Xizang（Tibet）Plateau was the main cause for high SOC and weak daily variation insummer at Waliguan.The similar effect of transportation was obtained at the Mauna LoaObservatory.The distribution characteristics of SOC increasing with height in the tropospheredetermined the difference of SOC between East China and West China.
