Abstract: A formula with second-order perturbation correction is obtained for the calculation NMR coupling constants by means of the unit operator consisting of non-orthogonal LMO and adjoint basis sets The electron densities at the carbon and hydrogen nucleus and the NMR coupling constants ~1J_(CH) for the CH_4, C_2H_4 and C_2H_2 were evalnated respectively. It has been shown that ~1J_(CH) is principally determined by bonding and anti-bonding LMO of the C-H bond and approximately proportional to the product of electron density at the carbon and hydrogen nucleus.