Abstract: Based on the regional observations of the undercompaction and the abnormal formation pressure occurring in the source rocks of Late Jurassic and Early Cretaceous layers and of other data in Jiuxi Basin, the authors suggest that the undercompaction and abnormal pressure of argillaceous of this area is mainly caused by compaction unequilibrium and by hydrocarbon generation, while the dehydration of clay mineral and aquathermal pressuring effects are not important. The abnormal pressure with pressure coefficients about 1.2-1.3 of this area can be the driving forces of oil primary migration. The combined effect of abnormal pressure plus tectonic stress can creat microfracture. After analysis the diameters not only of the source rock' s pore and of hydrocarbon molccullar's but also of the pollen's and spore's from crude oil, the authors suggest that the pore of the source rock can allow hydrocarbon migration. Because the diameters of the pollen and spore from oil are much larger than the pore size of the source rock's, the microfracture should be the important passage of oil primary migration. This paper also deals with the problem of the phases in primary migration of petroleum. The composition comparison between the extracts of the rocks and the cruds oils shows that the saturated hydrocarbons which arc among the least water-soluble, are enriched in cruid oils. At the same time, the estimation of the hydrocarbon abundance in the pore fluid from the pyrolysis data and the porosity decrease through oil window supports that the solubility of the hydrocarbon in the solution would reach 7188-82250ppm if the migration phase were molecular solution. This ideal solubility is obviously much higher than the actual solubilty, so it is impossible that the molecular solution is the principle mode of primary migrarion of pertoleum. At the end of this paper, from the comparison of the critical heights of different oil traps, secondary migration conditions of this area are discussed. The importance of hydrodynamic action in the Baidong- Danbei hydrodynamic trap is also analysed.