This paper presents an application of double Fourier series (DFS) algorithm for analyzing harmonics of a modular multilevel converter (MMC). By considering the effect of arm inductor, the application of DFS for calculating harmonics in the MMC is made. The DFS is then verified by comparing with simulation results used the fast Fourier transform (FFT) algorithm. Subsequently, the DFS will be used to analyze the effect of number of output voltage levels to the distribution of harmonics and total harmonic distortion (THD) in the MMC. The analytical results are performed in the Matlab programming in order to verify the theoretical analysis.