The organic solar cells (OSCs) with the structure of ITO/ZnO/C₇₀/rubrene/M oO₃/Al were fabricated to study the performance improvement of rubrene/C₇₀ based OSCs with ZnO is as cathode buffer layer.Simultaneously,the mechanism of the OSCs was studied by inserting ZnO with different thicknesses as cathode buffer layer．As can be seen from the experimental results,the characteristic parameters of the OSCs,including the sho rt-circuit currentdensity(Jsc),open-circuit voltage(Voc),filling factor (FF),power convers ion efficiency (PCE) and series resistance (Rs),show a regular change with different ZnO thicknesses.The PCE of the OSCs is increased with the increasing thickness of ZnO when the thickness of ZnO is smaller than 53nm.The PCE of the device is as high as 1.13% with 53nm-thick ZnO while the J sc,Voc,FF and Rs of the OSCs are 2.82mA·cm－2,0.84V,45.86% and 66.2Ω·cm²,respectively.The PCE of the OSCs is decreased with the increasing thickness of ZnO when the thickness of ZnO is larger than 53nm.Compa red with the device without ZnO,the J_sc,V oc,FF and PCE of the best OSCs are enhanced by 49%,29%,17% and 117%,respectively．Rs is reduced by 56%.