The law of gravitation is used widely. However, it is still unsolved how these two objects attract each other. Hypothesis, such as graviton, universal repulsion, sub-photon sea etc., all are of defects, having not explain the law of gravitation successfully. This paper assumes: (1) micro particles with a certain speed, moving like thermal motion, are distributed in pure space;(2) the objects are not continuous at the micro structure, there being gaps between the particles composed of the object, allowing part micro particles passing through;(3) momentum passed from particles to the objects is proportional to mass of the objects. Using theory of exchange of momentum between objects and micro-particles, the source of gravitational attraction and formula are explained reasonably. Gravitational attraction is an equivalent expression of the force close to each other between and on the line of the two objects, coming from exchange of momentum between objects and micro-particles. The constant of universal gravitation G is accurate only for near earth astronomical body. The gravitational attraction between two objects is affected by intermediary between them. There is no graviton.
Keywords: Gravitational attraction, Graviton, Universal Repulsion, Photon, Micro-particle, Momentum