Abstract: Occurrence and development of winter-time natural convection cooling effect in the fractured-rock embankment of Qinghai-Tibet highway were investigated using the governing equations based on air-flow function for natural convection in porous media with variable permeability.It was shown that winter-time natural convection in the fractured-rock embankment began to occur in the side slope portions and developed from two side slope portions to the middle portion of embankment with lowering of surface temperature.A natural convection index for representing the cooling effectiveness of winter-time natural convection in the fractured-rock highway embankments was introduced.There were three regions of the variation of the natural convection index with thickness of layer in the highway,i.e.,zero region,rapid increase region and no appreciable increase region.Thicknesses of fractured-rock layer in highway embankment corresponding to the initial point and end point of the rapid increase region were defined as the minimum and maximum thicknesses of fractured-rock layer constructed,which triggered winter-time natural convection,respectively.Evaluating fractured-rock layer thicknesses of highway embankment with various grain sizes was performed.