In the present study, density of abundance of spiders explored and dominant species determined in the cotton major fields i.e., Ardebil, Fars, Golestan and Tehran provinces in 2003-2004. Through regular weekly, specimens collected by shaking bushes and they were carried out for the further study to the Araneae laboratory. A total of 632 specimens were classified in 76 species, 59 genera belonged to 19 families. Twenty seven species and 15 genera of them were new records for spider's fauna of Iran. The collected spiders counted and frequency of each species and dominant species determined. The following species Thanatus formicinus (Clerck), Cheiracanthium pennyi (O.P. Cambridge), Aulonia albimana (Walckenaer), Neoscona adianta (Walckenaer) were dominant species in Tehran, Ardebil, Fars and Golestan cotton fields, respectively.