Abstract: For long time,it is believed that there is a conformity between Cretaceous Qingshankou Formation and Quandou Formation of Songliao Basin with continuous transition.Based on detail analyses of stratigraphic correlation,lithology,and lithofacies,especially with the increasing seismic data to interpret the five seismic profiles across the different structures of the basin,it is proposed there is an unconformity between the two formations resulted from uplifting movements.The T₂ seismic reflection is defined as a contact boundary between the two formations.It is suggested that the unconformity would exert influence on the migration and accumulation of petroleum.So that effort should be made in searching stratigraphic oil/gas pools and related structural traps above or below the unconformity.