Abstarct: Horizontal Gravitational separators are often used for separating oil and other existent compounds in oil well from water. In this research numerical simulation for gravitational separators using computational fluid dynamic has been done to evaluate and improve their efficiency. Software package GAMBIT 2.4.6 was used for geometry and mesh generation and software package FLUENT 6.3 was used for simulating oil/water or grease/water two phase flow. In simulation, mixture multiphase model and k-ε turbulence model was used. Generally oil/water separator and grease interceptor were evaluated as two instances of gravitational separators. Oil/water separator was evaluated with mixture of 20 percent oil and 80 percent water, whereas mixture in use on grease interceptor assessment only had 0.11 percent grease diffused in water. Simulation results in both of gravitational separators had good relative agreement with experimental results. grease interceptor evaluation results for mixture of only 0.11% grease in water using mixture multiphase flow model illustrate that mixture multiphase flow model can be appropriate for two phase flows with very low volume fraction second phase. In addition, effective changes on separating efficiency are evaluated and final improved model produce from composition of most effective changes, in order to improving efficiency of separator system.