  Neutrino oscillation is the new physics beyond standard model. In this paper
we have revisited the issue of neutrino oscillation in simple and delicate
manner. Starting from the fundamental quantum mechanical treatment for the
neutrinos, we calculated the probabilities for the neutrino oscillating within
three flavors. Normal and inverted hierarchy have been taken into account for
the solar and atmospheric cases, in vacuum and matter. The probabilities have
been plotted for all the cases and discussed.
