The present disclosure relates to a modified epoxy resin having a weight average molecular weight of 5,000 to 25,000 and a polydispersity index of 5.0 to 20.0, and comprising (1) an epoxy-derived unit and (2) a modifier-derived unit, a preparation method thereof, a composition comprising the same, and uses thereof. When the epoxy composition comprising the modified epoxy resin according to the present disclosure is cured, the epoxy composition has a lowered coefficient of thermal expansion (CTE), i.e., improved thermal expansion properties, due to curing-induced phase separation (morphological properties) into an epoxy region and an acrylic region. The modified epoxy of the present disclosure and the epoxy composition comprising the same are suitable as adhesives for semiconductor packaging.