Two-phase flow instabilities in parallel boiling channels under natural circulation condition were experimentally studied at low pressure (0.2-0.6 MPa). The parallel boiling channels composed of two different form channels, namely a 3 x 3 rod bundle channel and a tube channel, were used as the testing section. In the present experiment, the flow oscillation behaviors can be classified into three typical modes based on their frequency spectrum characteristics: (1) small amplitude out-of-phase flow instability induced by the type-1 density wave oscillations in parallel boiling channels, (2) large amplitude out-of-phase flow instability with reversal flow induced by the occurrence of geysering in tube channel and (3) a kind of compound flow instability superposed by geysering and natural circulation instability. The influence of thermal hydraulic parameters on the evolution process of above three flow instabilities were analyzed. The marginal stability boundaries presented by the subcooling number (N-sub) and phase change number (N-pch) at different system pressures are obtained and all unstable flow occurs in the negative region of the outlet equilibrium quality in the present experiment. Besides, the effects of asymmetric heating between two parallel channels on NC flow instability were also studied. The asymmetric heating can change the NC flow distribution in parallel boiling channels and reduce the system stability. The occurrence of out-phase oscillations in parallel channels can be suppressed with the increasing of heating power asymmetry. (C) 2018 Elsevier Ltd. All rights reserved.