The thermodynamic efficiency of Organic Rankine Cycle (ORC) obtained from experimental systems has been obviously lower than theoretical results. One of the reasons is that some elements affecting the efficiency, such as pressure losses in evaporator, condenser and pipes, have not been deeply discussed and optimized. The present manuscript detailed and examined the influences of pressure losses on ORC thermodynamic efficiency. Pressure losses in ORC system were divided into three categories: High Pressure Loss of the first category (HPL1), High Pressure Loss of the second category (HPL2) and Low Pressure Loss (LPL). A formula was developed to describe the relationships between three categories of pressure losses and ORC thermodynamic efficiency. Through analyses of the formula, it is found that the adverse impacts of pressure losses on thermodynamic efficiency are very different, mainly depending on the characteristic specific volume of working fluid in certain state. Because the very different specific volume, under a same value of pressure loss, LPL mostly leads to the largest thermodynamic efficiency drop of ORC system, HPL1 does less and HPL2 does the least. The influences of LPL are normally 3-21 times larger than that of HPL1, and 30-50 times larger than that of HPL2. The conclusions will serve as a guide for engineering applications of ORC system.