Abstract The prime objective of this work is to investigate the effects of different parameters on the steady state and transient behavior of a double-channel two-phase natural circulation test loop. In this paper, to investigate the stability of natural circulation system, the linear stability method (also known as frequency domain method) is used. First, the steady state parameters of the natural circulation system in equal and unequal heating power conditions for each channel are obtained. Then by using the steady state parameters, the stability of a two-phase natural circulation test loop by decay ratio (DR) stability criterion is studied. The steady state results are in good agreement with experimental data under low and high power conditions. In equal and unequal heating power conditions, the predicted behaviors by the represented model agree well with experimental data. The stability map is obtained for this loop. Like single channel natural circulation loops, two unstable regions (Type-І and Type-ІІ) exist in the double-channel two-phase natural circulation test loop. Results show that in a constant heating power by increasing the inlet sub-cooling, the extent of stable region reduces. Finally, the effects of various parameters on the stability of the system in the form of the stability map are investigated.