Hydrogenated microcrystalline silicon thin films with different volume fractions of nanocrystallites (Xc) were prepared by catalytic chemical vapor deposition (Cat-CVD) at different pressures. The temperature dependence of dark conductivity, sigma (d)(T), was measured for films with different Xc. A simulation Of sigma (d)(T) was carried out based on a simplified energy band model. Around room temperature, the fitting data show that thermionic emission of carriers over the potential barriers dominates the electrical transport for samples with lower Xc. However, for higher Xc, tunneling through the potential barriers is the fundamental conduction mechanism. In the temperature range of 100-210 K, films with lower Xc follow the relation ln sigma (d)(T)similar toT-(1/4) more closely than do samples with higher Xc. It was found that oxygen contamination contributed 0.15 eV to the film activation energy.