Abstract: Freeway should be, and has been the safest highway compared to other types of highway due to its highest design standard. Whereas these years Chinese government was forced to pay more attention to freeway safety problems due to its leading fatality rates despite its luxury infrastructure criteria. Research on crash rates and speed has been achieved by many people, but paper about the relationship between crash rates and average speed difference between cars and large vehicles on freeway kept rare. This research explored this relationship using three-year crash data and traffic flow data of one freeway of Beijing suburban section. From the result it can be concluded that speed difference and crash rates are correlative, and similar empirical functions have been obtained using compound, growth, and exponential curve to fit. The general law extracted from this paper showes that with the increasing of the average speed difference, the crash rates increase rapidly.