The objective of this diploma thesis is the investigation of factors that influence the severity of road accidents with one or more vehicles. Severity has been chosen to be expressed as the rate of number of persons killed and seriously injured divided by the number of slightly injured persons. The method of log-rate analysis was chosen for the statistical analysis of data of road accidents for the period 1999-2008 in Greece. For this purpose four different mathematical models of severity were developed, for accidents with all involved vehicles, for single-vehicle accidents without a pedestrian, for accidents with two and more involved vehicles and for single vehicle accidents with a pedestrian. In the mathematical models it was examined qualitatively and quantitatively the impact of various factors on the severity, which are related to the road, vehicle and involved person characteristics. Those concern type of region, intersection, weather and lighting condition, vehicle type, age and collision type. Analysis has led to a series of conclusions such as that single-vehicle accidents are 2.4 times more serious than accidents with two or more vehicles and the severity of accidents for the pedestrians is 1.2 times higher than that for the drivers and 1.4 than that for the passengers.