To describe possible effects of medium, besides the hamiltonian introduced by Mikheev, Smirnov and Wolfenstein (MSW), in principle, there may exist new terms in the evolution equation for the neutrino oscillation due to the environment. Considering a two-energy-level quantum system: ν1 − ν2, two-species neutrino system which is embedded in a matter environment, we solve the neutrino evolution e...