To improve interlaminar shear strength (ILSS) of ramie fiber (RF) reinforced polypropylene (PP) composites, graphene oxide (GO) with different particle sizes (ca. 1, 5 and 40 μm) are employed to treat the RF surfaces. Untreated and GO-treated RF/PP composites are then prepared by hot press. The combined effect of the GO size and content is examined on the ILSS of the RF reinforced PP composites. Moreover, the wrinkle morphology of GO surfaces is considered to play a mechanical interlocking role in improving the interfacial bonding between RFs and PP matrix, thus enhancing stress transfer from the PP matrix to the RFs. Consequently, the observed maximum improvement of about 40% in the ILSS of the PP composite by GO treatment of appropriate size and content is reasonably interpreted mainly via GO aspect ratio, GO aggregation and PP crystallinity.