Lorentz transformation of three-qubit Greenberger-Horne-Zeilinger (GHZ) state is studied. Also we obtain the relativistic spin joint measurement for the transformed state. Using these results it is shown that Bell's inequality is maximally violated for three-qubit GHZ state in relativistic regime. For ultrarelativistic particles we obtain the critical value for boost speed which Bell's inequality is not violated for velocities smaller than this value. We also show that in ultrarelativistic limit Bell's inequality is maximally violated for GHZ state.