Gravitational instantons are non-compact complete hyper-Kahler 4-manifolds with curvatures. They are introduced by Hawkings as the building blocks of his quantum gravity theory. The gravitational instantons are classified by their volume growths into types , , , , and , . In this talk, I will give a short proof of the Torelli theorem of gravitational instantons of type , namely their corresponding hyper-Kahler triples are completely determined by the cohomology classes of their hyper-Kahler triple. The proof is inspired by the SYZ mirror symmetry of log Calabi-Yau surfaces. This is a joint work with T. Collins and A. Jacob.