In this paper we present clearly a set of postulates on probabilities which are the basis of the nonlinear master equations and multivariate linear master equations appeared in non-equilibibrium statistical physics. By using the methods of probability theory, we deduced these master equations from our general postulates. We further discussed the conditions under which the master equations are consistent with the macroscopic kinetic equations. Finally, we discussed the fluctuation in Brusselator model using multivariate linear master equation approach and made a preliminary explanation on the appearance of dissipative structure through fluctuation.