An experimental study is performed to investigate the characteristics of vortex shedding behind a circular cylinder with serrated fins using hot-wire anemometer. Strouhal numbers which are calculated using outer diameter of a circular cylinder with serrated fins are higher than that of a circular cylinder. Fin thickness and pitch are closely related with vortex shedding frequency and play increasing or decreasing vortex shedding after transient Reynolds number. Strouhal numbers using effective diameters which are proposed in this paper agree with that of a circular cylinder. After transient Reynolds number, a trend of Strouhal number can be estimated by checking the ratio of effective diameter to inner diameter.