Leaf hoppers are important pests of crops including cotton. This research was carried out during 2014- 2015 to identify the main leaf hopper species collected in the cotton fields of Golestan province and to study population of cotton green leafhoppers dominant ones. The specimen were collected and brought to the laboratory. The results revealed that following the species had the highest population in cotton fields:1- Asymmetrasca decedens (Paoli,1932). 2- Psammotettix alienus (Dahlbom, 1851).3- Emposca decipiens (Paoli, 1930). 4- Jacobiasca lybica (Bergevin & Zanon, 1922). A. decedens was determined as dominant species. The peak of activity this insect was late in June to late in August in the cotton feilds. The biological characteristics of A. decedens were stuied in the laboratory condition (24 ±1 °C. and 75 ± 5 % R.H) 25 – 35 days with egg incubation period of 7 – 9 days and nymphal duration of 10 – 15 days. The longevity of adults was 8- 11 days. This pest had 5-6 generation per year and over wintered as adult under weeds. The main important cultivated host plants of this pest are cotton, potato, faba bean, rapeseed, soybean and cucurbit and its wild hosts are black night shade, pigweed, mallow and goose weeds.