The Theory of Observational Relativity, the theory of OR for short, is a new discovery and a new theory, which has revealed the root and essence of relativity: All relativistic effects or relativistic phenomena are observational effects and apparent phenomena rather than the objective and true physical reality. In particular, the whole theoretical system of OR has generalized and unified Newton’s mechanics and Einstein’s theory of relativity, integrating such two great theories in physics into the identical theoretical system under the identical axiom system. The theory of OR is divided into two parts: the theory of inertially observational relativity (IOR); the theory of gravitationally observational relativity (GOR). The theory of GOR takes the three principles of GOR as its axiom system: (1) the principle of GOR equivalence; (2) The Principle of GOR covariance; (3) the principle of the invariance of information-wave speeds. Based on the three principles of GOR, by following or by analogizing the logic of Einstein’s general relativity, the author has established the whole theoretical system of GOR, including the field equation of GOR and the motion equation of GOR. GOR’s field equation has generalized and unified Einstein’s field equation and Newton’s field equation (i.e., the Poisson equation form of Newton’s law of universal gravitation). GOR’s motion equation has generalized and unified Einstein’s motion equation and Newton’s motion equation (i.e., the second law form of Newton’s law of universal gravitation). The theory of GOR has proved an important theorem: the theorem of Cartesian spacetime which suggests that the objectively real spacetime could never be curved. So, spacetime is not really curved. Finally, the theoretical system of GOR has generalized and unified Newton’s theory of universal gravitation and Einstein’s theory of general relativity. It suggests that the theory of GOR is logically consistent not only with Einstein’s theory of general relativity but also with Newton’s theory of universal gravitation. Such logical consistency and strict correspondence show that the theory of GOR is logically self-consistent, and confirm the logical rationality and theoretical validity of the theory of GOR.