In order for different kinds of neutrino to transform into each other, the eigenvalues of energy of neutrino must be different. In the present theory of neutrino oscillations, this is guaranteed by the mass differences between the different eigenstates of neutrino. Thus neutrino cannot oscillate if it is massless. We suggest an explanation for neutrino oscillations by analogy with the oscillation of quantum two-state system, where the flipping of one state into the other may be regarded as a process of quantum tunneling and the required energy difference between the two eigenstates comes from the barrier potential energy. So neutrino with vanishing mass can also oscillate. One of the advantages of the explanation is that neutrino can still be described with Weyl equation within the framework of standard model.