This study was conducted in Keban dam lake Cemisgezek region during 2000-2001 fishing season. In the study, the effect of mesh size on catching efficiency was analyzed. For this reason, a total length of 800 m gillnet was used, particularly two monofilament gillnets with 50, 60, 70 and 90 mm mesh size, each being 100 m long. During the research, Cyprinus carpio and Squalius cephalus was indicated with own species names, which are caught on 22 hunting attempts. Barbus esocinus, Barbus capito pectoralis and Barbus mystaceus species regarded as Barbus sp., Capoeta trutta and Capoeta umbla species regarded as Capoeta sp. The most frequently caught species in the research were Capoeta sp. on 50 and 70 mm mesh sizes, Barbus sp. on 60 mm mesh size and Cyprinus carpio on 90 mm mesh size. The species that is least frequently fished on all gillnets was Squalius cephalus. When the catch per unit effort was analyzed taking into account all species, it was observed that with 83.1 g m-1 catch per unit effort, 60 mm gillnet was more efficient than all other gillnets. This gillnet was followed by 79.9, 41.0 and 31.8 g m-1 catch per unit efforts for 50, 70 and 90 mm mesh sizes, respectively. In conclusion, it has been found out that in the study region, gillnets with 50 and 60 mm mesh sizes are more effective than gillnets with 70 and 90 mm mesh sizes.