Starting from optical Dirac equation, an alternative form of Maxwell's equations, I introduce a statistical operator and derive the optical analogue of von Neumann equation, which is the dynamical equation of the operator. I also found one of its stationary solution, a thermal state. According to the asymptotic analysis of the thermal state, optical coherence is sustainable at the zero temperature, while unsustainable at the high temperature limit. We can regard this as decoherence of classical optical field.