  In this paper, the Lorentz transformation of entangled Bell states seen by a
moving observer is studied. The calculated Bell observable for 4 joint
measurements turns out to give a universal value,
$<\hat{a}\otimes\vec{b}>+<\hat{a}\otimes\vec{b}'>+\lang
le\hat{a}'\otimes\vec{b}>
-<\hat{a}'\otimes\vec{b}'>=\frac{2}{\sqrt{2-\beta^2}}(1+\sqrt{1-\be ta^2})$,
where $\hat{a}, \hat{b}$ are the relativistic spin observables derived from the
Pauli-Lubanski pseudo vector and $\beta=\frac{v}{c}$.
  We found that the degree of violation of the Bell's inequality is decreasing
with increasing velocity of the observer and the Bell's inequality is satisfied
in the ultra-relativistic limit where the boost speed reaches the speed of
light.
