Abstarct: Nowdays with increase the number of motor vehicle in world, number of accident significantly consideration also increase. High cost of fatal accident (F), personal injury (PI) and property damage only (PDO) that burden to society caused improving road safety an important issues to be considered in transportation and traffic. Identification of factor involved in transportation accident and countermeasure against these factors are amonge the most important measures to improve the safety of road. Number of road accident in curve three times the number of accident in direct routes. This subject indicated horizontal curves are from accident prone in the road. Due to the number of accident in horizontal curve, its seems necessary to provide an index to evaluate the safety of curve. An index that can be used for identification the unsafe curves and performance countermeasure in order to improving safety. In this study, intend with collecting the geometric and traffic characteristics road of the Shahrood to Azadshahr, provide an index to evaluate the safety of horizontal curves. In this regard, measured the lateral acceleration in the horizontal curves in this road with different speed by an accelerometer. For modeling used the factor contains curve radius, maximum lateral acceleration, difference between used and posted speed in curve, ratio of maximum lateral acceleration to theory lateral acceleration, standard deviation of lateral acceleration and standard deviation of lateral acceleration change, to achive the safety index using generalized linear models and have been proposed several modeles. Afterward according to their comfortable lateral acceleration tolerance and lateral acceleration of vehicle caused reversal, provide four level of safety contain absolutely safe, safe, low safety and uusafe, for identification safety index. According to this index that its lateral acceleration is hess than 0.35g is in absolutely safe area, between 0.35g-0.55g is in safe area, between 0.55g-0.7g is in low safety area and more than 0.7g is in unsafe area. Study on curve of Shahrood to Azadshahr roads, indicate that 60% of examined curve have low safety, that needs appropriate countermeasure for improving safety of these curves.