  We have studied the relativistic effects on the mean spin entanglement of two
massive Dirac particles using the simultaneous eigen-spinors of the
Foldy-Woutheysen mean spin operator and the Dirac Hamiltonian. We have obtained
the transformation matrix from the spinor with specific momentum to the spinor
with a transformed momentum under an arbitrary Lorentz transformation. Using
the transformation matrix we have shown the consistent monotonic behavior
between the concurrence and the maximum value of Bell parameter in Bell
inequality of transformed spin states.
