The dispersion state of surface Ti species is one of the most important factors affecting the polymerization properties of $MgCl_2$-supported heterogeneous Ziegler-Natta catalysts. In this paper, ethylene-propylene copolymerization was carried out using a novel Ziegler-Natta model catalyst with "isolated" and "clustered" $TiCl_3$ molecules on $MgCl_2$. At a lower ethylene/propylene feed ratio, the dispersion state of the Ti species strongly affected both the composition and sequence distribution of the copolymers. The "isolated" Ti species gave a relatively random copolymer, and clustering of Ti species enhanced the bulkiness. The addition of an external donor basically deteriorated the random copolymerization character of the "isolated Ti species but the degree of deterioration depended on the type of donors. On the other hand, the balance between the monomer feed and consumption dominated the copolymerization performance at a higher ethylene/propylene feed ratio to obscure the effects of the dispersion state.