Abstract: The principle of relativity requires that the expressions for particle＇s momentum and energy take the same functional form in all inertial frames. We present a general proof that the relativistic energy has to be proportional to the moving mass. This result is a self-consistent requirement of relativity. It does not dependence on the specific form of linear space-time transformations,nor does it require the postulation of an invariant speed of light.The proof further implies that conservation of mass in general follows from conservation of energy.

Key words: special relativity, momentum and energy, principle of relativity, mass-energy relation