The fauna of soybean leafhoppers and the biology of dominant species were investigated in Karaj during 1992-1994. For identification of species the male genitalia and other taxonomic characters were used. In order to find the duration and number of nymph stages, leaf cages were used on the plants. A thermograph was used to study the effect of temperature on nymphal stages in greenhouse. To study biology of E. decipiens in field four cages prepared and the trials has done inside them, and weekly all its life cycle observed. The following six species from four genera were identified: Empoasca decipiens Paoli, Macrosteles quadripunctulatus (KIrschbaum), M. laevis (Rib). Psammotettix alienus (Dahlbom), P. Striatus (Linne), Neoaliturus tenellus (Baker). Our investigations showed that Empoasca decipiens was the dominant species with the highest abundance. This specie is one of the important pests in many parts of Iran where oilseed crops are planted. The results showed that the soybean leafhopper over winters as adult under the debris of host plants and weeds. The first generation completes their life cycle on weeds before planting soybean. The second, third, and fourth generation lived on soybean. The time taken for the first, second, third, fourth and fifth nymphal stages were 6-7, 5-6, 2-4, 4-6 and 6-8 days, respectively. Adult longevity was 17-19 days, so that duration of first generation on weeds was 54-66 days. Our experiments showed that, at 18 degrees C condition, the first stages have been the maximum nymph period and at 23 degrees C condition, have been the lowest period.