Considering size of the country, growth of population, increasing vehicle ownership and so, increasing interchanges of inner and outer zones, traffic incidents are one of the most important difficulties of transportations. To remove this difficulty, recognizing causes and factors effective on accidents and determining the amount of their effect on traffic incidents should be of main priorities for organizations such as Roads & transportation and police. Totally, the main factors effective on accidents can be divided into four groups: Road (geometrical charactenstics, traffic flow features), vehicle (vehicle ability, technical fault) human (driver characteristics, driver behavior, and his mental & physiological ability) Environment (weather). These factors are joined chainly. These factors have counter. Effects on traffic incidents and if one of them becomes instable, the cycle will be unstable too, and intensification of this situation causes accident.On recent years, significant researches have been done all over the world about having relation among the number of accidents and main factors effective on accidents mentioned before, but in Iran, because of lack of accessibility to accident statistics, these type of researches have been done rarely. Here, in this article, we try to introduce effective factors on accident all over the world and also, Iran according to exact statistics and results of these researches will be remedy for related authorities to prioritize their activities about transportation safety.