The bulk polymerization of styrene catalyzed by Nd(P-204)(3)/MgBu2/HMPA (hexamethyl phosphoramide) was carried out in capped glass tubes. The effects of reaction conditions on polymerization conversion and molecular weight in the range of high conversion were investigated, The molecular weight of the resultant polymers is dramatically high and its distribution is relatively narrow. The polymerization process demonstrates the feature of living polymerization and auto-acceleration phenomenon. The auto-acceleration phenomenon is attributed to the non-instantaneous formation of the active species. The experimental data suggest that chain transfer to MgBu2 is one of the factors governing the molecular weight development. A mechanism of polymerization is presented with the chain transfer process incorporated.