Reactive liquid polymers (RLP), carboxyl-terminated butadiene-acrylonitrile copolymers (CTBN), epoxy-terminated butadiene-acrylonitrile copolymers (ETBN), amine-terminated butadiene-acrylonitrile copolymers (ATBN) of different molecular weights (from 2200 to 6500 g/mol), and a nonfunctional butadiene-acrylonitrile copolymer were synthesized and characterized. Selection of end groups of RLPs and the synthesis were discussed. Phase behaviors of CTBN, ETBN, and NFBN in the RLP/DGEBA system were investigated regarding to the molecular weight, molecular weight distribution, chain composition, and end groups with a light transmission method. These experimentally determined phase behaviors will be related to those of the RLPs in the systems to be modified. The effect of molecular weight of RLP on the viscosity was also studied.