Properties of the tactic and atactic hydrogenated ring-opened poly(endo-dicyclopentadiene)s and features of the tungsten imido/phenolate-catalyzed stereoselective ring-opening metathesis polymerizations (ROMP) were studied. Several tungsten( VI) imido phenolate complexes were synthesized and exhibited moderate ROMP activity in the presence of n-BuLi. W(=NPh)((R)-(+)-5,5', 6,6'-Me-4-3,3'-t-Bu-2-biphenolate)(2) was found to be effective for cis-, isoselective ROMP of endo-dicyclopentadiene (DCPD), while W(=NPh) Cl-4 center dot Et2O promoted cis-, syndioselective ROMP. On the other hand, W(=NPh)(2,6-Me-2-phenolate) 4 provided atactic poly(DCPD). Isotactic, atactic, and syndiotactic hydrogenated ring-opened poly(DCPD) s were characterized well by various methods for the first time. Both tactic polymers were shown to be crystalline polymers by means of DSC, WAXD, and TEM measurements. In contrast, atactic poly(DCPD) was an amorphous polymer. The crystallization rate of the syndiotactic hydrogenated poly(DCPD) was significantly higher than that of the isotactic polymer.