There are by now only one or two examples to show the initiation of coronal mass ejections (CMEs). In this paper we present a clear instance of a CME initiation. The enhanced disturbance was seen as the slow upward motion in segments of a prominence. Four hours later, the prominence was accelerated to about 60 km s−1. It greatly expanded, showing twisted structure, in later one hour. The CME onset was characterized as the disrupting and opening of the prominence. During the onset phase of the CME, a nearby small prominence erupted. It appeared that the global instability responsible for the CME was triggered by the initial mass ejection of the first prominence.