The magnetic field dependence (0-13.1T) of the lifetime of the chain-linked triplet biradicals, composed of xanthone ketyl and xanthenyl radicals, has been studied as a function of chain length. Upon increasing a magnetic field, its lifetime increases steeply to the respective maximum value at ca. 2 T. Upon further increasing the field, the lifetime of a long chain biradical decreases slightly, whereas those of all other biradicals are almost constant. Chain length dependence of the biradical lifetimes in a low magnetic field (0.1 to ca. 2 T) are discussed in terms of the spin-lattice relaxation (SLR) due to the dipole-dipole interaction between two radicals and the spin-orbit-induced intersystem crossing from triplet states of biradicals. A slight decrease in the lifetime, observed for a long chain-linked biradical in high magnetic field, is explained by the SLR due to the anisotropic Zeeman interaction.