Oncolitic limestone was firstly discovered in the Qingshankou Formation of Cretaceous in Songliao Basin. It occurs primarily in finegrained sandstone, siltstone and the mudstone. Based on the metamorphic features, the oncolites from the study area can be divided into three types: annular oncolite, stalagmite oncolite, and gingershaped oncolite. Most oncolites are composed of a core and multiring lamina, but minor amount doesn't have distinct core or monolayer lamina.Research indicated that the oncolites formed mainly in the underground shallow bay environment of lake delta front. The penecontemporaneous deformation of the oncolites is attributed to growing of topographic slope and deltaic front. The fact that the oncolites occur in the second and third sections of the Qingshankou Formation indicates a transitional period from humid to dried and hot climates, and therefore, the rock can be considered an important index of paleoclimate changes.