It is well-known fact that gears are damaged from severe wear more than tooth breaking. The reason of wear is elastic deformation which is occurred by high contact pressure between gears. Therefore, determination of contact pressure and elastic deformation is rather significant. Contact pressure is called Hertz pressure in gears. Distribution of pressure and deformation and cylindrical contact area is obtained by using elasticity theory. With developed program, the effects of module, pressure angle, profile shifting and gear ratio on Hertz pressure and elastic deformation are investigated. When transmitted moment is increased, Hertz pressure and elastic deformation are increased in both external and internal gears. However; gear ratio and module increase, Hertz pressure and elastic deformation decrease in external gears. In internal gears effect of module is similar to external gears; while gear ratio increases Hertz pressure and deformation increase as distinct from external gears.