In many applications the flow that past bluff bodies has frequency nature (oscillated) and it is not uniform. This kind of flow has effects on the formation of vortex shedding behind bluff bodies. In this paper the flow around a circular cylinder was numerically simulated. The effects of frequency variation at inlet flow on the vortex shedding frequency were investigated. The transient Two-Dimensional Navier-Stokes equations were employed to compute the unsteady laminar free stream flow over a circular cylinder. A time series of analysis for the formations of vortex shedding behind the circular cylinder was performed under uniform (Frequency=0) and oscillated flows (Frequency=0.1, 1, 10, 60 and 100Hz) at Re=300. Then the value of amplitude for one of the oscillated flows was changed. The global quantities such as drag coefficients and Strouhal number variables were compared for the above range of conditions for both uniform and oscillated flows. Results show that by increasing the inlet flow frequency, Strouhal number increases slightly and the oscillation amplitude of drag coefficient increases considerably. However the mean sizes of drag coefficients do not vary in all computations. Also the effect of variation in velocity amplitude on vortex shedding frequency and drag coefficient value were studied.