To improve the interfacial compatibility between graphene and poly(methyl methacrylate) (PMMA), amino-functionalization was applied to the graphene oxide via chemical modification using ethylenediamine (EA) and 1,6-hexanediamine (HA). Experimental results showed that with the incorporation of only 1 wt % amino-functionalized graphene oxide (AGO), the thermal stability, mechanical properties, and the dispersibility of fillers in the nanocomposites were improved significantly. The effect of the alkyl chain length of the diamine was also considered.