In the investigation of the course of degradation and build-up reactions during the decomposition of the tert-butyl perbenzoate (TBPB) (at eight different concentrations) in isotactic polypropylene (iPP), it was found that, at the beginning of the peroxide decomposition at all investigated peroxide concentrations from 4.62-200 mmol/kg iPP, the degradation reactions of iPP prevailed. At the TBPB concentration of less than or equal to 37.0 mmol/kg IPP during the whole period of peroxide decomposition, degradation reactions leading to a lower of molecular mass of PO prevailed. But at higher peroxide concentrations of TBPB greater than or equal to 74.4 mmol/kg iPP and at the later stage of peroxide decomposition, a predominance of the build-up reactions, that is, an increase the molecular mass, was observed. The degradation and build-up reactions were determined from the measurements of the melting-flow indexes of the peroxide-treated iPP samples. The reaction mechanism of the degradation and build-up reactions in iPP is discussed.