Chemical enhancement is an important mechanism in surface enhanced Raman spectroscopy (SERS).It is found that mildly reduced graphene oxide (MR-GO) nanosheets can significantly increase the chemical enhancement of the main peaks by up to one order of magnitude for adsorbed Rhodamine B (RhB) molecules, in comparison with the mechanically exfoliated graphene.The observed enhancement factors can be as large as ~103 and show clear dependence on the reduction time of graphene oxide, indicating that the chemical enhancement can be steadily controlled by specific chemical groups.With the help of X-ray photoelectron spectra, these chemical species are indentified and the origin of the observed large chemical enhancement can thus be revealed.It is shown that the highly electronegative oxygen species, which can introduce strong local electric field on the adsorbed molecules, are responsible for the large enhancement.In contrast, the local defects generated by the chemical reduction show no positive correlation with the enhancement.Most importantly, the dramatically enhanced Raman spectra of RhB molecules on MR-GO nanosheets reproduce all important spectral fingerprints of the molecule with negligible frequency shifts.Such a unique noninvasive feature, along with the other intrinsic advantages such as low cost, light weight, easy availability and flexibility, makes the MR-GO nanosheets very attractive to a variety of practical applications.