A novel poly(methyl methacrylate) (PMMA) nanocomposite containing dispersed inorganic nanoribbons [ZnO–0.15Zn(CH3COO)2–0.85H2O] was prepared by free radical polymerization of methyl methacrylate in the acetone solution. Experimental results showed that inorganic nanoribbons were uniformly distributed in and bonded to the PMMA host matrix without macroscopic organic–inorganic phase separation. It was found that the thermal stability and glass-transition temperature of the nanocomposite films increased effectively with increasing inorganic content at low content and remained above 1 wt% inorganic content. These results suggest the network formation because of the strong interaction between the inorganic nanoribbons and the polymer matrix, which induces the mobility restriction of polymer chains. The characteristics of the one-dimensional inorganic nanoribbons we used here may play a key role in the formation of the “cross-link” networks and in the decision to lower the content of the inorganic nanoribbon additive.