Gears are used for power transmission between two axes that they have often mechanical advantages like increasing or decreasing the torque from one shaft to another or the rotational speed. Spur gears are the most important tools for power transmission in the industry that can be considered the most common components of the transmitter force among the mechanical systems. Like other parts of a system, Gears consume a large amount of power and they may damage seriously such as wear, fatigue, failure. So the improvement in the design and modification of tooth profile of gears can significantly reduce friction loss and wear the gear teeth and therefore it effect on increasing life, maintaining the quality of performance and Cost reduction. As is commonly the case in the analysis of most tribological components, the key parameters of interest in gears are the lubricant film thickness, the dimensionless film parameter, and the coefficient of friction. The first two parameters are important in terms of reliability and damage, while the third parameter is a measure of the efficiency of the gear set in terms of the required power. In indusrial applications, the friction coefficient between the moving contacting surfaces is of vital importance in terms of power loss. Another influential parameter in lubricanted tribosystems is the roughness of the surfaces i.e. the micro geometrical irregularities of the surfaces. Gears usually operate under the mixed-lubrication regime. Hence, it is always of interest to predict the portion of load carried by asperities as well as friction coefficient. The aim of this research is to present a model that takes into consideration the addendum modification, surface properties, gear geometry, loading conditionand velocity condition as the input and predicts the film thickness, friction coefficient along the line of action (LOA). In this model, the contact of gear teeth at each point along the line of action is represented by the contact of two cylinders, where the radii of these cylinders vary along the line of action. The contact model for rough surfaces introduced by Greenwood and Williamson (1996) assumes that the asperities deform elastically, however, in reality when the contact pressure exceeds the yield strength of the material, elastic-pelasticdeformation occurs. The separation between two rough surfaces is given by the film thichness which would exists between two surfaces under the same conditions of load, speed and lubricant.in this research the model of Zhao et al of rough surfaces in contact has been combined with Moes' central film thickness equation and load sharing concept of Johnson et al use to predicting the film thickness, friction coefficient load carried by film and load carried by asperity a long line of action. Finally subsurface stresses are obtained using the coefficient of friction and hertzian contact model. The purpose of this study is showing the influence of addendum modification on performance of spur gears and the effect of that on Power distribution, friction coefficient and Load tolerated by the lubricant film and roughness. This model can be used as a rapid prediction for the modified gear performance. Keywords: Spur gear, addendum modification, Elastohydrodynamic mixed lubrication, friction coefficient, wear rate