Abstract: An epoxy resin oligomer is provided. The epoxy resin oligomer is obtained by the reaction of at least a first reactant and a second reactant, and the molecular weight of the epoxy resin oligomer is between 3000 and 9000, wherein the mole ratio of the first reactant and the second reactant is between 1:0.9 and 0.9:1, and each of the first reactant and the second reactant is a compound having two polymerizable groups.