Abstract: Two 3rd grade sequences, SY1 and SY2, are identified in Sanya Fm in Qiongdongnan basin through analyzing lithology, logging and seismic reflection signatures and by using concept of sequence stratigraphy. Sandbodies including "downcutting valley, basin floor fan, slope fan and lowstand wedge" are discovered in the LST (lowstand system tract) of these two 3rd grade sequences. These LST sandbodies were deposited on the Eogene chasmic stage sediments and had favorable reservoir conditions. The above thick claystone in ingression and highstand system tract might supply enough oil sources and act as good cap rocks at the same time. The LST sandbodies of Sanya Fm in Qiongdongnan basin have good reservoiring conditions, thus have great significances to petroleum exploration in this area.