We study the effect of incubation period on epidemic spreading in the Barabasi-Albert scale-free network and the Watts-Strogatz small world network by using a Suspectable-Incubated-Infected-Suspectable model.Our analytical investigations show that the epidemic threshold is independent of incubation period in both networks,which is verified by our large-scale simulation results.We also investigate the effect of incubation period on the epidemic dynamics in a supercritical regime.It is found that with the increase of incubation period Ω,a damped oscillation evolution of ρΤ(the ratio of persons in incubated state)appears and the time needed to reach a saturation value increases.Moreover,the steady value of ρI increases and approaches to an asymptotic constant with the value of Ω increasing.As a result,the infected ratio ρΤ decreases with the increase of Ω according to a power law.