Two-site model analysis was carried out for C-13 n.m.r. of polypropylene polymerized by Ziegler-Natta catalyst with external alkoxysilane donors. Four effects of the external donors were found : selective poisoning of symmetric site; transformation of a symmetric to an asymmetric (but fluctuating) site; increase of whole isotacticity due to that of polymers obtained by the fluctuating site; and increase of isotacticity of the polymers obtained by the fluctuating site when the concentration of external donor is high enough.