Recent Solar Dynamic Observatory observations reveal that some coronal mass ejections (CMEs) consist of multi-temperature structures: a hot channel and a cool leading edge (LE). The channel first appears as a twisted loop and lies along the polarity inversion line; it then gradually develops into the less twisted semicircular structure during the impulsive phase of CME acceleration. In the meantime, the hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME leading front. Here, we study two well observed CMEs, which occurred on 2010 November 3 and 2011 March 8, respectively. We find that, the hot channel rises earlier than the first appearance of the CME leading front and the onset of the associated flare, and that the speed of the hot channel is always larger than that of the leading front. Further differential emission measure analysis shows that the channel often has significant emission at high temperatures. We thus conclude that the observed hot channel is likely to be magnetic flux rope that acts as a driver of the CME acceleration in the early phase.