The effect of preshearing resin mixtures prior to casting on the structure and properties of in situ polymerized poly(methyl methacrylate) /clay nanocomposite panels was investigated. The preshearing was performed with a mechanical stirrer and controlled by varying mixing time. The structure, thermomechanical, and optical properties of the panels prepared with different preshearing times were analyzed by XRD, TEM, DMA, and UV/visible spectrophotometer. The properties of the panels increased with preshearing time because of improved intercalation and exfoliation of the clay. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2957-2960, 2008