The variation of Lyapunov exponent of Brusselator and Oregonator with controlling parameters of systems has been calculated. It is determined that the limitcycles of the two models are all produced by Hopf bifurcation according to the theory of stability of Lyapunov exponent.The conclusion of this calculation is the same as that of the analysis of linear stability for the behavior of Brusselator bifurcation. By analyzing the chemical background of Oregonator it is demonstrated that the major factors havingeffect on B-Z system bifurcation are the producing ability and producing rate of bromide ion. So all factors having effect on the producing ability and producing rate of bromide ion may affect the state of system. Therefore the effect of organic substrates, the catalyst of metal ion, pH and temperature ect. on B-Z reaction can be explaioned.