The nanoporous TiO2 films utilized in dye- sensitized solar cells ( DSSCs) possess a large surface- to- volume ratio, which facilitates the adsorption of sensitizing dye and the recombination due to the high density of surface traps. In this paper, nanosized PbS was fabricated on the TiO2 films. The components of the modified TiO2 film were studied by X- ray diffraction ( XRD) and electron probe microanalysis ( EPMA), while the structure of the film was characterized with BET physisorption and high-resolution scanning electron microscopy ( HRSEM). The results showed that the PbS particles were 2-3 nm and discrete on the surface of TiO2. Diffusion photovoltage ( PV) spectroscopy was employed to study the charge separation and diffusion processes inside modified and unmodified TiO2 films. The diffusion PV signals revealed that the traps on the surface of TiO2 were extremely reduced due to the presence of PbS. The results of transient photovoltage and back I- V characteristics showed that the back reaction, that is, the recombination due to the reaction between an electron on TiO2 and the hole- transporting media, was retarded significantly.