The external magnetic field effects (MFE) on the dynamics of chain linked singlet (S) and triplet (T) biradicals are briefly reviewed. MFE on the lifetime and yield of biradicals are interpreted in terms of the hyperfine (hf) mechanism and hf-relaxation mechanism of the radical pair model. Chain length dependence of these MFE is attributable to the energy gap between S and T stated of biradicals arising from electron exchange interaction. Lifetime and yield change of biradicals attributable to T-S level crossing is observed in many systems. The S-T degeneracy of biradicals occurs when the interradical distance becomes longer than 1 nm or so.