Polymer/graphene (oxide) nanocomposites exhibit enhanced mechanical properties at low volume fractions of graphene-based nanofillers. An understanding of the reinforcement behaviour was developed through the investigation of interfacial interactions between graphene oxide (GO) nanoplatelets and polymer matrix (PLLA, PCL, PS or HDPE) by combination of microstructure characterization and micromechanical modeling methods. The interfacial interaction determines the degree of dispersion of GO in polymers, interfacial adhesion strength as well as reinforcement efficiency, which can be tailored by the surface chemistry of GO and functionality of polymers. Homogeneous dispersion of GO nanoplatelets with high aspect ratios was found in PLLA and PCL matrices, as well as in lower polar polymer PS due to the preferable interactions between the aromatic rings and the graphene layers, while stacked GO layers with a lower aspect ratio were observed in HDPE matrix even with the presence of an organic compatibilizer. The theoretical elastic moduli of the four kinds of polymer/GO nanocomposites calculated by using the Halpin–Tsai model or a combination of Laminate theory and Mori–Tanaka model underestimated the experimental results. Considering the interfacial interactions, an effective volume fraction was introduced to the above composite models which interpreted the experimental data well. The interphase zone was thus quantified by using micromechanical modeling based on the measured mechanical properties of polymer/GO nanocomposites.