A copolymer is provided, which includes a chemical structure of: R is C1-6 alkylene group. T is a terminal group including C1-6 alkyl group, C3-6 cycloalkyl group, or C6-12 aromatic group. l is 0.05 to 0.3, m is 0.1 to 0.2, n is 0.5 to 0.8, l+m+n=1, x is 40 to 100, and o is 10 to 120. An epoxy resin composite is also provided, which includes 100 parts by weight of epoxy resin and 1 to 20 parts by weight of toughness enhancer such as the above copolymer.