In many cases, aphids are the most important pests of alfalfa. Regarding the importance of the aphids and little knowledge about their biological control on alfalfa in Iran , it was essential to perform a more complete investigation on identification of alfalfa aphids and their natural enemies in Isfahan . This investigation was conducted to identify alfalfa aphids and their natural enemies, study on population fluctuation of their dominants species and to investigate the effect of alfalfa strip-harvesting on their population density in Isfahan . In order to collect alfalfa aphids and their natural enemies, sampling was done from about 30 alfalfa fields in Isfahan province in different intervals for two years. Investigation on population fluctuation of alfalfa aphids and their natural enemies and also the effect of alfalfa strip-harvesting on population density of aphids and their natural enemies were carried out in a 2 hac. of alfalfa field. In this study 4 species of alfalfa aphids and 59 species of their natural enemies including 46 predators, 11 parasitoids and 2 parasitic mites were identified. Population density of Acyrthosiphon pisum and Therioaphis trifolii increased in June and then decreased in July and August but reached at peak in October and November. Population density of alfalfa aphid natural enemies were increased followed by increase of alfalfa aphids density. Mass multiplication of Hippodamia variegata , Nabis . and Geocoris . occurred in October and November and Mass multiplication of Deraeocoris ., Orius . and parasitoid wa occurred in June. After harvesting of tall alfalfa strips in alfalfa strip-harvested field, aphid natural enemies such as H. variegata , Nabis ., Deraeocoris . and Orius . moved to adjacent taller alfalfa strips and significantly increased on them but the movement trend of Geocoris . from harvested to adjacent taller alfalfa strips, were not in a logical order. In many cases the number of Acyrthosiphon pisum and Therioaphis trifolii in alfalfa conventionally harvested field were more than strips with same height of alfalfa in strip-harvested field. The effect of natural enemies in reduction of Therioaphis trifolii were more than reduction of Acyrthosiphon pisum. From the results of this study we can conclude that in contrast to the conventionally harvested alfalfa field, in strip-harvested field the density of aphid natural enemies were more and aphids