The composite solid electrolytes, NaSn2F5, dispersed with submicron size Al2O3 fillers of various concentrations and particle sizes have been synthesized through mechanochemical milling technique. X-ray diffraction and microstructure results indicate the biphasic nature of the composite materials. The transport properties of the present composite materials have been investigated by means of impedance spectroscopy, and the results show that the improvement in conductivity increases with decrease in the particle size of the filler. An enhancement in conductivity of two orders in magnitude is obtained for NaSn2F5 with Al2O3 dopant concentration of 10 mol%. The activation energy responsible for conductivity relaxation, calculated from the modulus spectra, is found to be almost the same as the value obtained from temperature variation of dc conductivity. The scaling result of the imaginary part of the modulus shows the temperature-independent relaxation behavior. © 2012 Springer-Verlag.