A novel epoxy siloxane hybrid was prepared using epoxy siloxane monomers of 1,3-bis[2-(3-17-oxabicyclo[4.1.0]heptyl))ethyl]-tetramethyldisiloxane (BEPDS) with hydroxyl terminated hydrogenated polybutadiene (GI-1000), in different proportions. Apparent polymerization reactivity was decreased with increasing GI-1000 concentration but the normalized reactivity per epoxy group was slightly increased due to reaction between hydroxyl group and epoxy group. Increasing GI-1000 concentration showed significant flexibility improvement in epoxy siloxane hybrid. At 30 wt % of GI-1000 addition, glass transition temperature was decreased from 116 to 21 degrees C and shore D hardness was decreased from 75 to 46.5% weight loss temperature of these epoxy siloxane hybrid was decreased with increasing GI-1000 concentration, whereas thermal discoloration was increased. LED encapsulation with this epoxy siloxane demonstrated no crack when GI-1000 was 30 wt % or more. (C) 2008 Wiley Periodicals, Inc.