Objective To study the proximal diameter changes of retinal blood vessel following branch retinal vein occlusion (BRVO). Methods Color fundus photographs and fundus fluorescein angiography (FFA) photographs of 48 patients with typical unilateral BRVO were analyzed using IMAGEnet software. The diameter of retinal artery (RAD) and vein (RVD) close to optic disc (within one DD from the optic disc) in four quadrants including the affected quadrant were measured with linear measuring tools.Results The proximal diameter of RAD and RVD in corresponding normal quadrants of the BRVO eye had no significant change comparing with the contralateral eye. The proximal diameter of RAD, but not RVD of the affected quadrant such as superotemporal （t=-2.342, P=0.026）or inferotemporal （t=-3.069, P=0.010）quadrant, increased remarkably. Conclusions In corresponding affected quadrant with BRVO, only RAD close to optic disc increases markedly, RVD has no significant change.