In this treatise,the initial cause of vertical and horizontal motions in fluids in gravitational field is investigated,which is the theoretical fundamental of the fluid mechanics,geology and meteorology.It is shown that there exists naturally the net lifting force in fluid acting on the fluid itself,expressed with the distribution of its density by(r2 g/8 p)/V2 p,named the UPDRAFT FORCE so as to be distinguished from the ordinary buoyancy term g(p-p0)/pfl.The up-draft force is the initial cause or driving force that we wish to find.For applications of the principle of up-draft force to the mantle in geology,a closed set of differential equations for the viscous fluid is presented,on which the proposed“up-draft force tectonic model”is based for explaining the genesis and driving force of orogeny,continental drift and collision,etc.These equations can be used in calculating or simulating the situations of flow in mantle if the data of variables are available. In this treatise, the initial cause of vertical and horizontal motions in fluids in gravitational field is investigated, which is the theoretical fundamental of the fluid mechanics, geology and meteorology. It is shown that there exists naturally the net lifting force in fluid acting on the fluid itself, expressed with the distribution of its density by (r2 g / 8 p) / V2 p, named the UPDRAFT FORCE so as to be distinguished from the ordinary buoyancy term g (p-p0) /pfl.The up-draft force is the initial cause or driving force that we wish to find. For applications of the principle of up-draft force to the mantle in geology, a closed set of differential equations for the viscous fluid is presented, on which the proposed “up- draft force tectonic model ”is based for explaining the genesis and driving force of orogeny, continental drift and collision, etc. The The equations can be used in calculating or simulating the situations of flow in mantle if the data of variables are available.