We study the interaction of magnons in dipolar spinor Bose-Einstein condensates in an optical lattice. By means of Holstein-Primakoff and Fourier transformations the energy spectra of the ground and the excited states is obtained analytically. Our results show that the collision of magnons is elastic which is expressed by the conservation of wave numbers in the process of collision. At last, we found that the interaction of magnons is attractive which tends to self-localization to form spin waves, i.e., a cluster of a macroscopic number of coherent magnons. Because of the attraction, the instability of spin wave brings about the existence of solitary wave.Of late, the dipolar spinor Bose-Einstein condensates (BECs) trapped in optical potentials [1-3] have been studied extensively. It offers new opportunity to confirm the dynamics of periodic structure in solid-state physics. In classical solid-state physics, the site-to-site interaction is caused mainly by the exchange interaction resulted from the direct Coulomb interaction among electrons and the Pauli exclusion principle. As a result, there are the phenomena of ferromagnetism or antiferromagnetism at temperatures below the Curie temperature. For the dipolar spinor BECs trapped in optical potentials, the internal degrees for the hyperfine spin of the atoms is freedom, which brings forth a rich variety of phenomena such as spin domains [4,5] and textures [6]. When the potential valley is very deep, the condensates at each lattice site would act as microscopic magnets and interact with each other through the long-range and anisotropic dipole-dipole interaction. These site-to-site dipolar interactions can cause the ferromagnetic phase transition [7,8] leading to a 'macroscopic' magnetization of the condensate array, the spin-wave-like excitation [7-10] and magnetic soliton [11,12] analogous to the spin-wave and magnetic soliton in a ferromagnetic spin chain. Therefore, the spinor BECs in an optical lattice offers a totally new environment to study spin 1105