Abstract: Flow, speed, and density are three basic variables that can be used to describe the traffic flow characteristics. The relationship among them is the foundation of the traffic engineering research. This paper started at the basic character of the traffic flow, deduced the theoretical formulas of the relationship between traffic density and vehicle speed according to the three phases, and further discussed the theoretical formulas of the relationship between traffic volume and vehicle speed, traffic volume and traffic flow density. The formulas accord with the curves of field survey results.