Flow-field study was conducted concerning vortex shedding from a circular cylinder with a slit. Experiments were carried out in a water channel and a low-speed wind tunnel with emphasis on the impact of the slit width on the quality of the vortex shedding signal measured. It is found that among the cases studied, whose slit widths ranging from 0 to 0.3 d, where d denotes the diameter of the circular cylinder, an optimal range of slit width, 0.1-0.15 d, was identifed, in which not only the vortex shedding signals show the best quality, but also the relation of the Strouhal number against the Reynolds number appears to be the most linear. By spanwise correlation of hot-wire measurements in the wind tunnel, it is further noted that in this situation the vortex shedding flow structures appear to be almost perferctly two-dimensional in the near wake region. Moreover, it is worthwhile to mention that for the cases of the circular cylinders with slits, the signal quality of vortex shedding reamins well acceptable for the Reynolds number as low as 2.4×103. Thus, the rangeability of flow measurement is increased significantly.