A full rang of behaviors of high-strength concrete box girder prestress with external tendons have been tested. The crack patterns, deflection, strain and stress distribution, prestressing loss and stress increment of external tendons have been measured. The results demonstrate that measured prestressing loss agrees well with the present highway bridge design code of China, besides, the shear lag effect in compressive flange and the stress increment in external tendons during the fully loaded range of the box girder also have been obtained from the experimental study and reasonable effective width for compressive flange relative to different stages has been proposed based on the measured results.