In this paper, we investigate the Lorentz invariance of the square root Klein-Gordon equation. This equation with interpreting ∣ψ∣2 as position probability density is not Lorentz invariance but we can make this equation Lorentz invariant with the help of new proper interpretation of the wave function. In this regard we consider the relation between the probability density and the wave function asρ=ψψ∗+(D^ψ)∗D^ψ and show appropriate selection of the linear operatorD^the square root Klein-Gordon equation will be Lorentz invariant. In fact the solutions of this equation is equivalent to the positive energy solutions of the Dirac equation. \end{abstract}
We have shown that the Born rule (ρ=∣ψ∣2) is inconsistent with Lorentz symmetry of the Salpeter equation (square root Klein-Gordon equation). So we find relativistic modification of the Born rule as ρ=2mc21(∣H−mc2ψ∣2+∣H+mc2ψ∣2) , which is consistent with Lorentz symmetry of this equation