The effect of a new bicomponent beta-nucleator composed of malonic acid (MA) and calcium stearate (CaSt) on the,formation of the beta-crystalline form in isotactic polypropylene (iPP) has been investigated. It has been found that the relative content of the beta-crystalline form (k value) of iPP notably increases with the addition of the MA/CaSt mixture, and it attains a maximum value of 0.94 at 0.15% MA/0.30% CaSt. For iPP nucleated with the bicomponent nucleator, the spherulitic size decreases continuously with increasing CaSt dosage. FTIR analysis has shown that cm "in situ" reaction may occur between MA and CaSt during melt blending, yielding an effective beta-nucleator (calcium malonate, CaMt). The beta-nucleation efficiency of an MA/CaSt mixture is higher than that of CaMt itself; this may be ascribed to greater dispersion of the "in situ" produced CaMt in the iPP matrix through the assistance of the CaSt residue and the stearic acid.