A strategy to prepare foamable isotactic polypropylene (iPP) was proposed by introducing crystal nucleating agent, 1,3:2,4-bis(p-methylbenzylidene) sorbitol (S20), and the foaming mechanism was also investigated. When a proper concentration (above 0.3 wt%) of S20 was added, iPP/S20 exhibited excellent foaming performance, yielding uniform cells with well-defined structures and higher cell density than those of neat iPP. The results of rheological analysis and TEM confirmed the formation of S20 nanofibrils in iPP matrix. The existence of these nanofibrils increased the melt viscosity of iPP and enhanced the bubble nucleation in the foaming process, and thus should be responsible for the improved foaming ability of iPP. Moreover, iPP/S20 could be foamed in a remarkably wide temperature range from 143 to 163 A degrees C. In comparison, another crystal nucleating agent, sodium 2,2'-methylene-bis-(4,6-di-tert-butylphenyl)-phosphate (NA40), contributed little to the foamability of iPP although it also induced the crystallization of iPP.