We investigated the stability of the liquid phase of 1-butyl-3-methylimidazolium iodide (hereafter abbreviated as [C(4)mim][I]) up to 16.7 GPa at room temperature. We observed a peculiar phase transition behavior in the [C(4)mim] [I] sample. In particular, a glassy state was formed at similar to 1.3 GPa; however, the reddish-brown precipitate was formed probably due to concentrated I-3(-)- or I-2(-) species that were formed above 12 GPa; [C(4)mim] [I] showed a pressure-induced partial crystallization from the glassy state. We concluded that the conformation of [C(4)mim](+) is essential in iterative modulation to control the environmental formation of iodide precipitate.