The spin Hamiltonian parameters g factors g(i) (i=x, y, z) and the hyperfine structure constants A(i) of vitamin B(12r) have been theoretically studied from the perturbation formulas of these parameters for a Co(2+)(3d(7)) ion with low spin (S=1/2) in rhombically distorted octahedra. The related crystal-field parameters are determined from the point-charge-dipole model and the local structure around Co(2+) in vitamin B(12r). The theoretical spin Hamiltonian parameters are in good agreement with the experimental data.