A new photopolymer satisfying both phase stability and high diffraction efficiency was successfully prepared using modified poly(methyl methacrylate) as a polymer matrix. In this work, we introduced methacrylic acid unit into the poly(methyl methacrylate) in order to enhance the phase stability without sacrificing the refractive index difference between polymer matrix and photopolymerized polymer. By the introduction of hydrophilic acid unit, optical transparency of the photopolymer was greatly enhanced and high diffraction efficiency near 90% could be obtained. There were remarkable differences in optical behavior among the polymer matrices with different concentration of acid unit and they were interpreted in terms of miscibility among polymer matrix, triethanolamine, and photopolymerized polymer.