This paper discusses the dynamic characteristics of plastic plates with bolted joints. The effects of tightening torque on the modal properties of the plates are investigated. Experimental and numerical modal analyses have been conducted on the plates made of acrylonitrile butadiene styrene (ABS), that are clamped by bolted joints. First, the effect of tightening torque on the vibration mode of the plates is investigated by experimental modal analyses. Modal testing has been conducted for various tightening torque values, and the relationships between the modal parameters and the tightening torques are discussed. Second, the effects of tightening torque on the vibration mode are studied by using analytical models for the bolted joints based on the finite element method (FEM). Based on the comparisons between the experimental and the numerical results, a modeling strategy for the boundary conditions between the plates is introduced and its validity is discussed. From both experimental and numerical studies, it is shown that the natural frequencies of the structures with bolted joints tend to converge to specific values as tightening torque increases. Moreover, it is also shown that when modeling the bolted plates by FEM, the inter-plate motion should be constrained by a boundary condition to properly suppress the out-of-phase motion of the plates.