An ultrathin SnO2 layer was introduced as a blocking layer in dye-sensitized solar cells (DSSCs) for improving conversion efficiency. The SnO2 blocking layer, composed of approximately 10 nm nanoparticles, was deposited on the fluorine-doped tin oxide (FTO) coated glass by SnCl4 treatment, a simple and inexpensive method. This SnO2 blocking layer increased the onset of dark current from FTO to electrolyte, suppressing charge recombination at the FTO/electrolyte interface. The cascading band structure of TiO2 and SnO2 increased the charge carrier lifetime, resulting in effective charge collection. Examining DSSC performance as a function of SnCl4 treatment time, a peak conversion efficiency of 7.33% was achieved for a SnCl4 treatment time of 15 min (SnO2 layer thickness similar to 12 nm), compared to 6.08% for an untreated cell. Also, compared to a TiO2 blocking layer, the SnO2 blocking layer showed a higher performance.