Modeling the I/O characteristic curves of the units is an important part of the unit commitment problem. Five models for I/O curves of units are presented and analyzed. Based on the IEEE RTS 96 system， the units with convex， non-convex and discontinuous I/O curves are scheduled respectively， and the results and computing times are compared based on different models in different cases. Furthermore， the number of integer variables， continuous variables and constraints based on different models are listed to analyze the differences in computing efficiency between the models， which is helpful in choosing the appropriate model according to the characteristic of unit for higher efficiency. The results show that the number of the integer variables affects the performance of the algorithm remarkably. For units with convex I/O curves， it is better to model the power by adding up the outputs in all the segments to avoid integer variables. For units with non-convex or discontinuous I/O curves， the assistant integer variables are inevitable and it is better to use a model with less continuous variables.