Abstract: The visual images of dryout, signals of flow, wall temperature, and pressure differences before and after the critical heat flux (CHF) were obtained in 5 mm clearances channels through natural circulation experiment, respectively. The qualitative and quantitative researches were conducted by applying nonlinear analysis techniques. The research results show that the power spectra of time series of pressure difference drop exponentially in semilogarithmic coordinates, the autocorrelation coefficient descends gradually, and the phase diagram exerts the characteristic of strange attractor, which proves that the nonlinear chaos characteristics exist in the natural circulation system when CHF occurs. During the initial period of CHF occurrence, the flow and the heat transfer represent periodic characteristics due to the flow oscillation and flow pattern changed periodically. With the increment of heat flux of heating channel, the periodicity decreases, while the randomness increases. A certain state of the system will be achieved, which embodies the characteristics of chaos movement.