The mechanisms of electron transport and back-reaction kinetics in dye-sensitized solar cell (DSC) were investigated by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). The DSCS with three different sizes of TiO2 particles were measured by IMPS/IMVS. The results indicate that the effective diffusion coefficient (Dn) increases along with the increase of particle size, while the electron lifetime (τn) and the electron transit time (τd) decrease with the increase of particle size under different induced-light intensities. The increase of Dn is attributive to the decrease of the film surface area. The decrease of τn is interpreted according to the frequency of electron trapping and de-trapping in defects. The decrease of trap concentration in TiO2 films leads to the decrease of τd.