Highly transparent optical resins based on poly(methyl methacrylate) (PMMA) and two cyclotriphosphazene derivatives which acting as halogen-free flame retardants were prepared. The refractive indices, visible light transmittance, water absorption, flame-resistant characteristic, and the mechanical properties of the resins were studied. Comparing with pure PMMA, the as-prepared resins with the two additives exhibited higher refractive index (n(d)), longer UV cutoff wavelength and better surface hardness. The water absorption of the resins was decrease with increasing of the additives. The visible light transmittance of all the samples adding Additive A was higher than 90%. When the weight ratio of additive/MMA was 20/100, the limiting oxygen index (LOI) of the samples were 26 (with Additive A) and 22 (with Additive B), respectively. The surface hardness was increased from HB to 2H with increasing the content of the additives, and the impact and the tensile strength changed little for the formulas of the additives lower than 30% by weight. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 727-734, 2011