Abstract: This paper conducts an analysis study on effect of the heat source flowrate on heat transfer characteristics of a preheater and system performance of a 250-kW level organic Rankine cycle (ORC) system. Refrigerant R245fa is used as working fluid with a constant flowrate of 11.58 kg/s. The operating pressures of the preheater/evaporator and the condenser are 1.265 MPa (i.e., evaporation temperature, T_R,eva, of 100 ^oC) and 0.242 MPa (i.e., condensation temperature of 39 ^oC), respectively. The analyzed heat source flowrate (m_w) is ranging from 9.39 kg/s to 27.39 kg/s with a constant inlet temperature of 133.9 ^oC. The efficiencies of the used pump, turbine, and generator are set to 90%, 80%, and 90%, respectively. The net power output is 243 kW and the system thermal efficiency is 9.46% under design conditions (m_w = 15.39 kg/s). For an off-design heat source flowrate, a new operating pressure of the preheater/evaporator will be chosen to meet that R245fa reaches the saturation liquid state at the outlet of the preheater and the saturation vapor state at the outlet of the evaporator, i.e., without superheating. The results demonstrate that (1) the operating pressure of the preheater/evaporator increases from 0.775 MPa to 1.675 MPa (i.e., T_R,eva increases from 79.3 ^oC to 113.2 ^oC) with an increase in m_w, ranging from 9.39 kg/s to 27.39 kg/s; (2) the higher m_w presents the better heat transfer performance of the preheater and the smaller requirement of heat capacity of the evaporator; and (3) the net power output (172 kW to 282 kW) and the system thermal efficiency (7.07% to 10.65%) increase with an increase in m_w, especially for m_w < 17.39 kg/s.