A hetero-quantum-dots (HQD) model for hydrogenated nanocystalline silicon films (nc-Si∶H) is proposed. The main contents of our model are: (i) the nanocrystalline grains and their amorphous conterparts have very different band gap and band structures. As a result, they form heterojunction like structures in the interface regions, where the band offset effects dramatically reduce the activation energy and the grains act like quantum dots; (ii) in the presence of an external field, the activated electrons in the quantum dots conduct via quantum tunneling through the interface barriers. By means of the HQD model, we have identified the conduction of nc-Si∶H as thermal assisted tunneling process. Our results show that there are two distinctive regimes for the conductivity of nc-Si∶H:(1) low temperature regime,where there is a simple activation energy ΔE; (2) high temperature regime, where ΔE is enhanced by the temperature effect of the electronic tunneling in the nanoscale particales. The theory is in good agreement with the experiments. Finally, we propose a perfect conductivity formula for nc-Si∶H films.