Graphene oxide (GO) emerges as a functional material in optoelectronic devices due to its broad spectrum response and abundant optical properties. In this article, it is demonstrated that the change of optical transmittance amplitude for monolayer GO (mGO) could be up to 24.8% by an external electric field. The frequency harmonics for transmittance spectra are analyzed by use of Fast Fourier Transforms to give an insight into the modulation mechanism. Two physical models, the electrical permittivity and the sheet conductivity which linearly vary as the electric field, are proposed to response for the transmittance modulation. The model-based simulations agree reasonable well with the experimental results.