A combined cycle gas turbine（CCGT） model based on conventional unit commitment（UC） is presented in order to achieve energy savings. The CCGT has various operation modes. It is difficult to model the multiple operating modes and mode transitions. Considering these difficulties， a model is presented which can handle the modes in transition as well as the transition cost very well. Furthermore， this model which uses the transition matrix is much more compact than those reported in previous publication. A single CCGT scheduling and IEEE 118-bus system including 12 CCGTs and 54 thermal units are simulated. The results show that the presented CCGT model can be easily combined with the conventional UC model， and it is effective to schedule the CCGT and deal with the mode transitions.