The modified formula for calculation of elasto plastic Hertzian contact stress was established. The formula was adopted to calculate the bonding strength of magnetic sputtered TiN coatings and compared with the experimental one. At the same time, the factors affecting the bonding strength of the coatings are discussed as well. As the results, the bonding strength of magnetic sputtered TiN coatings is greatly affected by the substrate surface roughness. In comparison with the other methods for measurement of the bonding strength of coatings, the current method has been found to be applicable and convenient for the bonding strength evaluation of coatings.