Failure of the steady solution of the master equation was analysed. It
was found that the state of sustained oscillation exists only when th
e ratio of photon decay rate gamma to injection late tau is satisfied.
Based on this understanding, a quantum mode micromaser theory was dev
eloped. The threshold nu(th) and photon distribution for pi and 2 pi m
ode were calculated. The instability of quantum mode was analyzed as w
ell.