In order to quantify the paramagnetic sources such as oxygen and deoxyhemoglobin in lung and other tissues with MRI of hyperpolarized helium and xenon nuclear spins it is important to understand the relaxation law of these hyperpolarized spins. By means of quantum mechanical calculations we found that the conventional spin relaxation equations are not valid for the hyperpolarized spins. We observed that that the hyper-polarization and spin-coupling result in the multipole spin orders. These multipole spin orders cross-relax each other. New longitudinal relaxation equations for the dipole-dipole coupled hyperpolarized spins at room temperature have been derived. These new equations predict that the longitudinal magnetization relaxation of hyperpolarized spins is, in general, bi-exponential decay instead single exponential decay. The potential applications of the findings in quantitative MRI with hyperpolarized helium and xenon are discussed. © 2006 IEEE.