Abstract: The term "massive sandstone" in this paper is designated to deseribe those sandstones which show no visible internal structures. X-ray radiography has provided to be a new method for the study of massive sandstones. Some of the massive sandstones show different kinds of stratification on X-ray exposures, but some of them remain structureless and homogeneous even under the radiation of X-ray. The former can be produced by vertical or lateral accretion and may be called "pseudomas-sive"; the latter are mainly formed by gravity flow and may be called "true massive". In the present paper, the principle and process of this method have been briefly introduced, and the genetic mechanisms of the two types of massive sandstones have been discussed in rather detail, exemplified by the Dainan Formation of northern Jiangsu, China. Finally, according to the associated sequences of sedimentary structures, several kinds of depositional environments which account for the formation of massive sandstones are recognized. The result of the study indicates that this method is of practical significance for identifying the sedimentary environments of reservoir rocks and for guiding hydrocarbon exploration.