In this paper the time dependent interactions between two viscous deformable drops that move under effect of buoyancy force are simulated numerically in three dimensions, and the effects of dimensionless parameters on film drainage are studied. Drops have different sizes so diameter of leading drop is half the diameter of trailing drop. The Navier-Stokes equations are solved with a finite difference-front tracking method. Bond number is considered to be large enough in order that drops can deform significantly. Important dimensionless numbers used here, are Bond (Bo) number, Morton (Mo) number, Reynolds number (Re), viscosity ratio (??) and density ratio ( ). The results obtained by Manga and Stone (1993), in the zero Reynolds number limit show that there are three methods for thin film drainage between two drops: 1-Rapid drainage in which the thinnest region of the film is on the axis of symmetry 2- Uniform drainage in which the film has a nearly constant thickness 3- Dimple formation The kind of fluid drainage that occurs between two drops depends on two dimensionless numbers which are Bond number and viscosity ratio. But the main factor in this issue is Bond number and for moderate Bond numbers, viscosity ratio may be effective. For all Bond numbers, the drainage begins with the first method (rapid drainage). In this case, the thinnest area of film is on the symmetry axis and the faster drainage occurs. When the distance between drops decreases (as time proceeds), drainage method changes to second and then third method. In a small Bond number, drainage is carried out only with first method. For large Bond numbers drainage changes to the second mode. Only for moderate Bond numbers all of the three kinds of methods can occur. The Reynolds number affects collision of drops more for small Bond numbers (more rigid drops) and the separation distance that are obtained from simulations do not agree whit the zero Reynolds number limit for small Bond numbers. Also the collision of two drops at the beginning of the movement is influenced by the Reynolds number. The thickness of the thin film decreases faster for low Bond numbers (1) compared to medium Bond numbers (10). For a moderate Bond number, dimple formation for ? = 1 occurs earlier. Also For greater viscosity ratio (? = 5) dimple formation occurs at a larger distance from the axis of symmetry. For all range of Reynolds numbers, rising velocity and thus Reynolds number decreases with increasing the viscosity ratio and increases with increasing of Bond number. Also drop deformation increases with increasing Reynolds number. Finally, the motion of four drops is simulated and the effect of increasing the number of drops on the drops motion is studied. Key Words: Collision, drops, gravity, buoyancy force, three dimensions