In this work, the dependence of the second-order magnetocrystalline anisotropic constant K2 on the minimum nucleation field Hminn has been investigated by the numerical method and the analytical formulation under the second-order approximation. It is found that the contribution of K2 t o H minn is very large at low temperatures for isotropic Pr2Fe14B magnets. Although the value of Hminn for Nd2Fe14B can b e obt ained using the analytical formulation under the first-order approximation, the value of Hminn for Pr2Fe14B is achie ved using the formulation under the second-order approxiamtion. Using the obtained value of Hminn, a good linear relation between coercivity and Hminn is found in the temperature range from 20 K to room temperature.