An indoor experimental test rig of parabolic trough collector (PTC)-based natural circulation steam generation system consisting in a thermosyphon loop was presented. A series of five heat loads (0.6-1.2 kW) were applied to investigate effect of heat load on thermal performance of the system. Effect of heat load on flow pattern, thermal efficiency and two phase heat transfer coefficient was discussed, respectively. An extended correlation equation was provided for two flow patterns, which is characterized by heat pipe thermal resistance. The critical heat pipe thermal resistance for flow pattern transition was ranged from 34.37 K/kW to 33.35 K/kW. Simultaneously, thermal efficiency shows a continuous increase as heat load kept rising. The effect of backflow was found to be negligible when heat load increased to 1.1 kW. Additionally, the average two-phase heat transfer coefficient in receiver also went up with the rising of heat load for the same flow pattern. Due to the flow pattern transition, which resulted in a dryness fraction drop in receiver, a maximum heat transfer coefficient of 285.86 W/m(2) K was obtained at heat load of 1.0 kW under a steam discharging pressure of 0.15 MPa. (C) 2014 Elsevier Ltd. All rights reserved.