In this paper, the relativistic harmonic oscillator equation which is a nonlinear ordinary
differential equation is investigated by Homotopy perturbation method. Selection of a linear operator,
which is a part of the main operator, is one of the main steps in HPM. If the aim is to obtain a
periodic solution, this choice does not work here. To overcome this lack, a linear operator is
imposed, and Fourier series of sines will be used in solving the linear equations arise in the
HPM. Comparison of the results, with those of resulted by Differential Transformation and Harmonic
Balance Method, shows an excellent agreement.