Abstract: Non-linear time series analysis on irregular complex flow oscillations of natural circulation system under rolling motion was studied. The correlation dimension, K₂ entropy and maximal Lyapunov exponent (MLE) were determined based on phase space reconstruction theory. The chaotic features of the irregular complex flow oscillations were analyzed by comparing with density wave oscillation and trough-type flow oscillation base on the results of geometric invariants. The results indicate that irregular-complex flow oscillations are typical chaotic oscillations, the interaction and feedback of thermal driving forces, flow resistance and additional forces caused by rolling motion lead to the emergence of chaos.