Lithium dendrite growth in Li/poly(ethylene oxide) (PEO)–Li(CF3SO2)2N (LiTFSI)–nano-SiO2/Li was examined using direct in situ observation under galvanostatic conditions at 60°C. Both the onset time of dendrite formation and the short-circuit time of the cells were extended by the addition of nano-SiO2 filler into the polymer electrolyte, of which an acid-modified nano-SiO2 filler was the most effective. The onset time was dependent on the current density in the range from 0.1 to 1.0mAcm−2. Li dendrite growth in Li/PEO18LiTFSI/Li at 60°C for current densities of 0.1 and 0.5mAcm−2 started at 125 and 15h, respectively. PEO18 LiTFSI with addition of 10wt% acid-modified 50nm SiO2 showed extended dendrite formation onset times of 250h at 0.1mAcm−2 and 32h at 0.5mAcm−2. The suppression of dendrite formation at the Li/PEO18 LiTFSI interface could be explained by enhancement of the conductivity and suppression of the interface resistance between lithium and the polymer electrolyte by addition of the nano-SiO2 filler. The electrical conductivity of 4.1×10−4 Scm−1 and interface resistance of 405Ωcm2 for PEO18 LiTFSI at 60°C were respectively increased to 7.2×10−4 Scm−1 and decreased to 77Ωcm2 by the addition of 10wt% acid-modified nano-SiO2.