Wo have formulated the statistical mechanics of magnetoactive plasma in the framework of irreversible statistical mechanics. Starting from the formal solution of the complete Liouville equation, we have obtained the one-particle distribution function. We have incorporated the quantum effects which become dominant at low temperatures by preparing the initial state using the Landau wave functions. Thus, the theory can be extended to low temperatures with a weak magnetic field. The microinstabilities that are usually discussed in plasma physics can be derived from the present results by taking proper limiting processes and this shall be discussed later on.