An extensive research is made on the chaotic phenomena in Chua's circuit. The double scroll can be obtained not only by changing one of R, C_1, C_2 and L, but also by changing the op-amp's voltage of the two-terminal nonlinear resistor. Moreover, in Chua's circuit there exists a rule,period-chaos-period plus(minus)l. This is confirmed by adjusting the op-amp's voltage, resistance, capacitance or inductance monotonously.