A finite difference scheme in cylindrical coordi- nates was used to study the three-dimensional (3D) motion of a vortex ring with swirl and the passage interaction between two vortex rings. For the 3D evolution of a single thin ring, the azimuthal perturbation modes grow linearly in the early stage. According to their growth rates, two bands of growing waves, which correspond to the first and the second radial model respectively, can be observed. The result is similar to the prediction of short wave instability theory for swirl-free vortex rings. For the passage process between two rings, re- sults show that the azimuthal velocity is in inverse proportion to radius while the azimuthal vorticity is in proportion to radi- us during the interaction.
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