An oxotungsten complex coordinated with O-S-O chelating bis(aryloxo) ligand, WO{S(4-Me-6-(BuC6H2O)-Bu-t)(2)}Cl-2 (1), was synthesized by the reaction of WOCl4 with 2,2'-S(4-Me-6-(BuC6H2OH)-Bu-t)(2). The structure of the complex 1 was determined by single-crystal X-ray analysis to have pseudo C,symmetry. The catalytic behavior of 1/AlEt3 system for the ring-opening metathesis polymerization of norbornene was studied. In this catalytic system, strong dependencies of cis/trans selectivity on the catalyst concentration and on the polymerization temperature were observed. With increasing catalyst concentration and with lowering polymerization temperature, the resulting poly(norbornene)s tend to have higher cis-regularity. Oppositely, trans-rich poly(norbornene)s were produced by the polymerization under low catalyst concentration and at high polymerization temperature. We propose that these observations might come from oxotungsten-aluminum interaction.