The electron diffusion length is directly evaluated from the dependence of photocurrent on the position of dye-adsorption front using the structure of selectively dye-adsorbed solar cell. By changing the thickness of the region without dye-adsorption, the thickness that the photocurrent starts to flow is defined as the electron diffusion length in nano-porous TiO₂ layer. This method is applied to investigate the effects of the electrolyte solvent and the annealing temperature on the electron diffusion length in nano-porous TiO₂ layer. It is suggested that viscosity and molecular size of solvent affects the diffusion of electron in TiO₂ nano-porous layer from the results that the electron diffusion length is 1.6 times longer in aprotic acetonitrile solvent than in protic ethylene glycol solvent. At 600°C annealing, the electron diffusion length shows the maximum of 27 ± 1 μm in LiI/I₂/acetonitlire at the incident photon flux of 2.2 × 10¹⁵ cm^-2 s^-1 at 540 nm. [DOI: 10.1380/ejssnt.2009.61]