Isotactic polypropylene (MI=7.7g 10 min. and M_w=150,000 g/mol) was chlorinated in the presence of ditertiary butyl peroxide(DTBP) in chlorobenzene. In this chlorination the degree of chlorination was always kept below 40% w/w. With increase of chlorine flow rate the reaction rate was increased, but the amount of unreacted chlorine escaping from the reactor was also increased, and the more DTBP added, the greater reaction rate resulted. Even at the same addition rate of chlorine the reaction rate was increased with initial IPP concentration In the temperature range 110-130℃, the reaction slowed down as the theperature was raised.