Abstract: For a nuclear check valve, the flow characteristics and mechanical properties of the primary components during the opening process were investigated. The commercial computational fluid dynamics code of ANSYS Fluent and user defined function (UDF) were used in combination to simulate the flows inside the valve. Furthermore, the fluid-solid interaction method was used at various valve openings to predict the mechanical behaviors of the valve components exposed to the fluid flow. The stress distributions over the valve body and the valve plug were obtained. The deformation of the valve body and the valve plug was quantified. The results show that with the increasing of valve opening, high velocity arises at the throat and the valve outlet. High pressure gradients emerge at the valve inlet. The largest deformation of the valve body is observed at the throat and deformation magnitudes fluctuate with the valve opening. The maximum equivalent stress is produced at the elbow area; it varies inversely with the valve opening. For the valve plug, the largest deformation is produced at the side close to the valve inlet. The maximum equivalent stress arises at the contact region between the valve plug and the spring. Different from its counterpart associated with the valve body, the maximum equivalent stress in the valve core increases with the increasing of the valve opening.