In this paper a comprehensive model of current conduction in polycrystalline silicon (polysilicon) based on the thermionic‐emission‐diffusion (TED) theory is developed, and on the basis of this model an expression for the effective majority carrier mobility &mgr;effis derived. This expression is quite general in nature and some thermionic emission (TE) theory based expressions for &mgr;effcan be obtained from it straightaway under certain simplifying assumptions. In addition, it helps in understanding the physical significance of the scaling factor used by earlier workers to explain their experimental results. Also, the experimental data on Hall mobility, which we obtained under an ohmic conduction regime in the 300–440 K temperature range for dark and illuminated conditions in lightly dopedn‐type polysilicon samples of different grain sizes, are presented and are interpreted on the basis of the TED model. Under strong illumination, the Hall mobility &mgr;HLwas observed to vary with temperatureTaccording to the relation &mgr;HL=aT−b, whereadepended on grain size and was found to be smaller for the smaller grain size. The dark mobility data fitted well into the TED‐based expression for &mgr;effconsidering the interface states associated with grain boundaries to be localized atEv+0.63 eV in the band gap. The analysis reveals that, generally, the scaling factor is needed if the effect of diffusion is neglected in comparison with the thermionic emission while in essence it is appreciable to be considered. However, in the TED model, as diffusion contribution in controlling the current transport across the grain‐boundary potential barrier is well taken care of, the scaling factor is not required.