Abstract: Room temperature tensile test was done to TC4 titanium alloy specimens with different diameters to study the influencing rules and mechanisms of specimen diameters on the tensile properties such as tensile strength,yield strength,percentage elongation after fracture and percentage reduction of area.The results show that the tensile strength and yield strength decreased as exponential function with the increase of specimen diameter,and percentage elongation after fracture and percentage reduction of area decreased as a simple linear function with the increase of specimen diameter.These phenomena may be caused by different microstructure and machining modes of TC4 titanium alloy specimens with different diameters.