A series of microcellular silicone rubber foams were prepared by supercritical carbon dioxide (scCO(2)). The effect of reinforcing agent (silica) on the rheological behavior, cellular morphology and nucleation of silicone rubber composites was investigated. The results show that the silica not only acts as reinforcing agent but also plays an important role in the cellular nucleation. When the content of silica increases from 40 phr to 70 phr, the range of the calculated surface tension of silicon rubber composites/scCO(2) (gamma(mix)) and radius of the critical nucleus (r*) are 158.95 similar to 1,092.74 nN/m and 14.45-99.34 nm, respectively. Meanwhile, aggregated silica has good heterogeneous nucleation as the diameter of aggregated silica particles is approximate to twice r*. The smallest cell diameter and the highest cell density of the silicone rubber foam can reach to 708 nm and 1.02 x 10(11) cells/cm(3), which indicate that the silicone rubber nanofoams can be obtained by means of the supercritical foaming technology. POLYM. ENG. SCI., 59:5-14, 2019. (c) 2018 Society of Plastics Engineers