Abstract: Two-Phase flow instability in natural circulation loops of China Advanced Research Reactor was investigated. The homogeneous flow model is used to establish the system control equations. Gear method is employed to solve the system equations documented in form of ordinary differential equations numerically. The boundaries of the instability of natural circulation loops were obtained. The variations of flux, pressure difference, wall temperature, fluid temperature and the boiling boundary were analyzed. Especially, the phase space trajectory of boiling boundary at various flux was discussed. The calculated results have great significance for the safety operation of China Advanced Research Reactor and its accident analysis.