Nano-composite polymer electrolytes are receiving attention as potential candidates to be used as electrolyte membranes in lithium polymer batteries and other devices. However, a survey of literature reveals that a systematic study of the effect of concentration and surface area of ceramic fillers on the conductivity enhancement of micro- and nano-composite polymer electrolytes is lacking. In this work, we have studied the thermal and electrical properties of the composite polymer electrolyte (PEO)9LiCF3SO3+Al2O3 incorporating alumina filler grains of four different sizes with different specific surface areas. The results show that the PEO crystallite melting temperature decreased by a few degrees in samples with fillers exhibiting a minimum for samples with high conductivity. The presence of the filler enhanced the ionic conductivity substantially above as well as below 60°C, and the nano-porous alumina grains with 5.8nm pore size and 150m2/g specific area and 15wt.% filler concentration exhibited the maximum enhancement. The observed conductivity enhancement has been attributed to Lewis acid–base type surface interactions of ionic species with O/OH groups on the filler surface, with an additional contribution below 60°C coming from the retention of an increased fraction of the amorphous phase due to the presence of the filler. The conductivity versus filler concentration curves exhibit two conductivity maxima which has been explained in terms of the surface interactions, blocking effect and grain consolidation. The conductivity enhancement appears to saturate beyond 100m2/g grain surface area.