The silica aerogels are super thermal insulating materials due to their extremely low thermal conductivities.The gaseous thermal conductivity in aerogels is much smaller than that of the gas in free space since the motions of gas molecules are greatly restricted by the fine solid matrix and extremely fine pore sizes in aerogels.This study presented several models to relate the pore sizes and the nanoscale solid skeleton with the gaseous thermal conductivity.The results show that the gaseous thermal conductivity in the silica aerogels decreases dramatically with reducing the pressure and pore sizes and with increasing the density.Thus，the gaseous thermal conductivity in aerogels at very low pressures is far smaller than that of the free gas at the ambient pressure.The nanoscale solid structure of the silica aerogels has a significant effect on the gaseous thermal conductivity，especially for pressures between 0.01×105 Pa and 100×105Pa.