Rift Valley Fever(RVF)is a mosquito borne disease of increasing global importance, affecting animals & human. Following the emergence of RVF virus in Khartoum State in 2007,this study was undertaken to assess ecological elements of mosquito related to transmission of RVF in Khartoum State. Emergence rates,Survival and parity rates were estimated. Survival of blood-fed female mosquitoes exceeded the intrinsic incubation period of the viruses inside mosquito vector. RVF virus was detected in Aedes vexans and Culex quinquefasciatus. Ae.vexans was suggestively the main vector of RVF during the last outbreak.We argue that two scenarios of transmission initiation might occur during the outbreak:1.Arrival scenario: where Ae.vexans had just arrived as adult mosquito and was infected with RVFV so horizontal transmission cycle initiated;or 2.Establishing scenario:that Ae.vexans was already well established in the area,then diapauses infected eggs were hatched and the vertical transmission cycle initiated. Ae.vexans might pass the virus to Culex via vertebrate host which established the zoonotic cycle,when the rainy season came mosquitoe density elevated transmission threshold of the outbreak