Niangniangshan alkaline volcanic complex developed in the late stage of Mesozoic volcanic activities in Ningwu volcanic structural depression was special type of rock association, In general, they were characterized by rich-potassium, LREE and other hygromagmaphile elements in geochemistry, This alkaline volcanic complex can be subdivided into two parts, belonging to the products of two sub-cycles of volcanic activities respectively, The lower part of this rock association bears pseudoleucite, characteristic mine- ra.l rich in potassium, and the upper part of this association bears hattyne, characteristic mineral rich in sodium, In this paper, we suggest that they came from a same magma chamber with compositional gradients by means of the petrologic, petrochemistry and trace elements geochemistry study. The crystal fractionation accounts for coming to compositional gradient in the chamber, but thermogravitational effect may play some role during magma differentiation, particularly for the generation of capping layer rich in volatile components, The mass balance calculations quantitatively modeled the crystal fractionation process in the chamber, and confirmed the results observed. That is to say, as a mineral of the lower density, leucite phenocrysts accumulated in the top part of the chamber, whereas those phenocrysts having higher density (such as pyroxene, hatiyne, plagi- oclase) settled downward in the chamber.