  In three generations or more, we derive the oscillation probabilities of both
Dirac and Majorana neutrinos relativistically by using the Dirac equation. We
present various oscillation probabilities for including wrong-helicity
neutrinos, right-handed neutrinos, and anti-neutrinos. We summarize the
relations between these probabilities. As neutrinos have finite mass, there are
two components for each chirality corresponding to positive and negative
helicities. We show that the probability is different for each component even
if neutrinos have the same chirality. The probabilities derived by the
relativistic equation depend on not only the mass squared differences but also
the absolute masses of neutrinos. Besides, the new CP phases appear in the
probabilities of oscillations with chirality-flip. These new CP phases are
equivalent to the Majorana CP phases in the case of Majorana neutrinos. We
investigate the CP dependence of oscillation probabilities in vacuum. There are
no direct CP violation in $\nu_{\alpha}\leftrightarrow \nu_{\beta}^c$
oscillations even if the flavors, $\alpha$ and $\beta$, are different as in the
same as two generations. In other words, the difference between the
CP-conjugate probabilities vanishes. However, in three generations or more, the
sine terms of new CP phases appear in the probabilities in addition to the
cosine terms. This is different from the result obtained in two generations.
Furthermore, the zero-distance effect does not appear in our formulation.
