The potential energy surface (PES) for the malonaldehyde intramolecular hydrogen atom transfer has been generated with full dimensionality by the modified Shepard interpolation method at the computational level of the second-order Moller-Plesset perturbation theory. The reference points have been set along the reaction path of H atom transfer (51 points), in a three-dimensional reaction space determined by geometrical features of the reaction path (219 points), and in the region of cis- and trans-enol isomerization reaction paths (428 points), so the resultant PES was generated in terms of ab initio data (energies, gradients, and Hessian matrices) of 698 reference points. Following trajectory simulations on the full-dimensional PES, the energy splitting of vibrational ground states due to tunneling was estimated by the semiclassical method of Makri and Miller [J. Chem. Phys. 91, 4026 (1989)]. The tunneling splitting was evaluated as 13.9 cm(-1), which is in good agreement with the experimental value of 21.6 cm(-1).