In this paper, the effects of different diameters of ZnO nanofibers prepared by electrospinning on the conversion efficiency of organic solar cells were studied. First of all, ZnO nanofibers with diameters ranging from 43 to 110 nm were prepared by electrospinning technique. Then, ZnO nanofibers with different diameters were added to inverted organic solar cells (ITO/ZnO∶ZnO nanofiber/PTB7∶PC70BM/MoO3/Al) as electron transport layer. Compared to the planar ZnO electron transport layer, ZnO nanofibers have the advantages of large specific surface area, increasing the ability of electron transfer and extraction and improving the photoelectric conversion efficiency of the device. It was found that the smaller the diameter of ZnO nanofibers is, the greater the efficiency of the cells will be. When the diameter of ZnO nanofibers was (46±5) nm and the receiving time was 30 s, conversion efficiency of the cell increased by 8%.