Abstract: A new self-consistent method for calculating the eigen-value of a nonlinear harmonic oscillator is proposed. The results show that the equivalent potential, the average value of coordinate operators, and the eigenvalue energy are in agreement with those by using the perturbation method in the case of small quantum number, indeed, the both are the same at zero quantum number.

Key words: quantum mechanics, self-consistent method, perturbation theory, nonlinear harmonic oscillator