Abstract: Generally speaking, during the seccodary migration of hydrocarbon, capillary power is the resistance of hydrocarbon migration when the rear-end curvature radius of the continuous oil (gas) phase is greater than the front-end's and is the driving force when the rear-end curvature radius is smaller than the front-end's because of the change of pore structure. Capillary power always tends to making the nonwetting phase occupy larger pore space. Under the joint action of buoyancy, hydrodynamic force and capillary power, oil (gas) moves intermittently. This has been verified in the physical simulation experiment.In fact, there are three types of capillary power. The first and the second types are parallel to the extensional direction of capillary, and the third is perpendicular to the wall of capillary and directs to the nonwetting phase. The third type of capillary power takes effect to increase the frictional resistance between the nonwetting phase and the throat wall, and is usually neglected in the media with wider throats (such as reservoirs). Hydrocarbon can migrate only when the driving force in the advancing direction surpasses the sum of the capillary resistance and the frictional resistance in this direction.A prevailing viewpoint says that capillary power is an important drivivng force during the primary migration of hydrocarbon from source rocks to reservoirs. So under the water-wetting condition, the oil (gas) phase will move spontaneously from small pores and thin throats to larger pores and wider throats. In this paper, the author studied this problem starting with the essence of capillary appearance and found that different types of capillary power act quite differently in source rocks comparing with in carrier beds. Because of the wider throats, the first and the second types of capillary power take main parts and the role of the third type of capillary power is often neglected in carrier beds. And in source rocks, the third type of capillary power increases greatly compared with the first and the second types because the capillary radius reduces all-roundly. This results in the steep rise of frictional resistance and makes it be the main resistance of hydrocarbon expulsion from source rocks. Therefore the viewpoint that "capillary power is a driving force for hydrocarbon expulsion" is a wrong conclusion due to unreasonable neglecting the third type of capillary power which plays a decisive role in source rocks. By studying the pore structure of argillutite, the author suggested that the pores in argillutite should be mainly "dead pores", and the hydrocarbon expulsion of source rocks should be eruptive surging through microfractures whose opening or forming and the happen of surging all have something to do with sudden and fierce tectonic movement.