A new method is proposed to determine the power-law order of gas-phase ethylene homopolymerization rate with time-varying activities over morphology-controlled prepolymerized catalyst at temperatures of 30-70 degrees C and ethylene concentrations of 120-900 mol/m(3). The prepolymerized catalyst was obtained by ethylene prepolymerization at mild conditions in heptane using a catalyst, in which TiCl4 was supported on spherical MgCl2 prepared by melt quenching of the MgCl2-ethanol complex. Triethylaluminum was used as a cocatalyst for prepolymerization and homopolymerization, During ethylene homopolymerization, the overall rate changed markedly with time, and two maxims in activity were observed in the absence of mass transfer effects. The order of power-law rate functions with respect to ethylene concentration was not constant and decreased with time on stream from about 1.5 to about 1.0 after about 4 h. These results show the presence of at least two types of catalytic sites having rates with different ethylene concentration dependencies and suggest that the two different types of catalytic sites have different deactivation behavior in this heterogeneous Ziegler-Natta catalyst.