For polyethylene produced over Ziegler-Natta catalysts in a semibatch slurry reactor, a kinetic model was used to predict molecular weight and polydispersity. A computer simulation on molecular weight and polydispersity was carried out with the changes of chain-transfer rate constants (k(trm), k(trh), and K-tra) and initiation rate constant (K-i). The effects of monomer concentration, aluminum alkyl concentration, hydrogen concentration, and temperature on molecular weight and polydispersity with respect to polymerization time were investigated.