Single-molecule magnets (SMMs) are materials showing magnetic bistability at the molecular level. This review provides a fundamental concept of the SMMs by reviewing the slow magnetic relaxations and the quantum tunneling of the magnetization (QTM) of a Mn12 cluster and lanthanoid(III)-phthalocyaninato double-decker complexes. In the latter part, the enhancement of the SMM properties utilizing the various kinds of dimerization method is reviewed. The dimerization by connecting the two double-decker SMMs with Cd(II) ions revealed that very weak SMM-SMM interactions could suppress the QTM. Covalently linked SMM dimer showed larger magnetic hysteresis than monomer complex did, indicating that magnetic properties were enhanced by the ferromagnetic SMM-SMM interactions. Finally, the supramolecular interactions in the solution were utilized for dimerization. The magnetic relaxation time (τ) of the supramolecular dimer became 1000 times larger than that of the monomer, showing the effectiveness of the dimerization for enhancing SMM properties.