This study is aimed at using thermal history control and nucleating agent to adjust the foaming behavior of isotactic polypropylene (iPP). Through the introduction of an annealing stage and a nucleating agent, the crystallization and rheological properties of iPP were changed, and its microcellular foaming behavior was tuned correspondingly. Rheological testing results verified the enhancement of melt strength through the presentation of the crystallization stage and a nucleating agent. After an annealing at 110 degrees C, 10 MPa, for 40 min, iPP was further saturated at 165 degrees C, 12 MPa for 30 min and then foamed, the cell density is 7.95 x 10(9) cells/cm(3), while the average cell size is 11.66 mu m. Furthermore, the synergy between the annealing treatment and an addition of 0.2 wt.% a nucleating agent could positively affect the heterogenous cell nucleating ability and melt strength of iPP, much smaller sized cells with relatively quite high cell density up to 10(13)-10(14) cells/cm(3) were induced.