Abstract: The aerodynamic forces on a circular cylinder in the critical Reynolds number range significantly influence its large-amplitude vibrations. The wind pressure tests are carried out in the present study to clarify the relationship between the wind pressure distribution and large-amplitude vibrations for a smooth circular cylinder in the critical Reynolds number range. The results show that the large-amplitude vibrations occur in the critical Reynolds number range. Meanwhile, the aerodynamic forces along the length of the cylinder are different when the vibrations occur. This difference implies that the unsteadiness in a flow state is the main reason of the large-amplitude vibrations of a smooth circular cylinder.