In this work, the effect of vapor chamber characteristics, the properties of its working fluid and the operating parameters on the vapor chamber performance are studied. Also, the effects of these parameters on the cooling process are considered. A three dimensional hydrodynamic model is used for solving the fluid flow through the liquid and vapor regions of the vapor chamber. The hydrodynamic model is coupled with a three dimensional thermal model to calculate the model temperature. The hydrodynamic model takes into consideration the circulation of liquid between the two wick regions. An implicit finite difference method is used to solve the numerical model and a validation of the numerical model is presented. The effect of porosity of the wick material, wick structure, solid wall material, working fluid, wick region thickness, vapor region thickness, power input, and heat transfer coefficient of the cooling fluid are taken in the study. Their effects on the heat pipe temperature, pressure difference of the heat pipe, liquid and vapor velocities and mass evaporated are studied. The results show that, to increase the cooling performance of the heat pipe, the porosity, wick thickness, power input, and vapor region thickness should be decreased and the heat transfer coefficient should be increased. To minimize the maximum pressure difference of the heat pipe, increase porosity, wick thickness, and vapor thickness and decrease heat transfer coefficient and power input. The study shows that the increase of wick thickness by a factor of four decreases the maximum pressure difference by about 75% and increases the maximum vapor chamber temperature 30%. It also shows that the vapor region thickness has an insignificant effect on the vapor chamber temperature and pressure. The increase of the heat transfer coefficient of the cooling liquid decreases its effect on heat pipe performance.