In this study, one-step approach to prepare stabilizer-free Ag-graphene nanocomposites using DMAc-assisted thermal reduction method with uniform distribution of "near spherical'' Ag nanoparticles (Ag NPs) in the range of 16-20 nm is reported. Interestingly, from the change of absorption spectrum as a function of reaction time, we observed that the characteristic absorption peak of Ag NPs shows no peak position shift in a quite long time without extra stabilizer while red-shift and broaden after continuous reaction. To explain this phenomenon, we further proposed a growth mechanism that CO, which is generated from reduction of functional groups on GO, adsorbed on the surface of Ag NPs and leaded to growth cease of Ag NPs into a narrow size distribution during the reduction of GO. Meanwhile, Ag NPs can catalyze the oxidation of adsorbed-CO to CO2 in the presence of O-2 which can easily desorb from Ag surfaces. Hence, after fully removal of functional groups on GO, continuous supply of CO was cutoff while the desorption of adsorbed-CO was still happening continually, so Ag NPs start to gradually grow and resulting in aggregation. Moreover, the dosage of less DMAc or more AgNO3 would cause the anisotropic growth and form multiply twinned structure of Ag NPs. Our study presents a useful understanding on the growth of Ag NPs on graphene.