Optical absorption enhancements of graphene by exciting graphene surface plasmon waves using nanogratings are studied. Based on temporal coupled mode theory, we show that the electron relaxation time of graphene and the geometry of the device play crucial roles in determine the absorption rate, and complete optical absorption is possible by adjusting the geometric parameters of the device to match the electron relaxation time of graphene. Moreover, the absorption spectrums can be dynamically tuned by varying the Fermi energy of graphene. Our study could benefit the development of graphene based photonic and optoelectronic devices, such as infrared photodetectors and bolometers. (C) 2015 Elsevier B.V. All rights reserved.