Hailstorm is a rather unusual weather phenomenon in Taiwan particularly in the summer season. The hailstorm event occurring over northern Taiwan on 2 July 1998 was investigated using conventional data, Doppler radar observations, and satellite cloud winds. Results showed that an upper-level cold vortex provided favorable conditions for the development and evolution of the storm. The storm appeared to be triggered by the low-level convergence associated with local circulations coupled with the upper-level divergence forced by a jet streak of cold vortex. Backing of winds and vertical shears with height provided by cold vortex appears to be instrumental for the westward propagation and intensi- fication of the convective system. A couplet of mesoscale vortices was observed in the low- and midtroposphere and was found to be generated by the tilting process on the vertical wind shear through a strong updraft in the convection.