Irreversible degree of heat transfer in organic Rankine cycle (ORC) is proposed in this paper. Based on a trapezoidal cycle (TPC) and its theoretical model, the model and mathematic formulas of irreversible degree of heat transfer is built. Thermodynamic relation between irreversible degree and cycle performance (thermal efficiency, net power output and exergy efficiency) is built and studied, which can be applied to the coupling optimization between cycle performance and heat transfer processes. Similar to net power output, there exists a kind of shift-curve of irreversible degree and the corresponding shift-temperature of heating fluid for working fluids, which indicates the shift of irreversible degree from having optimum condition with a minimum to monotonic decrease with heating fluid temperature. The range of irreversible degree and range of cycle performance according to irreversible degree are obtained respectively. Moreover, a model of working fluid with linear saturation line is proposed to study the general thermodynamic principles of TPC (or ORC) theoretically without the restriction of actual working fluids.