A robust optical strain sensor, called fiber Bragg grating, is employed to measure the effective cure shrinkage-a part of cure shrinkage that is accumulated only after the gel point-of a dual cure adhesive during each stage of curing-(1) UV-curing stage and (2) thermal-curing stage. Unique experimental setups and procedures are developed and implemented to cope with the technical challenges associated with each stage of curing, including gel point detection and uniform curing. The effective cure shrinkage is measured to be 2.1% and 0.78% for acrylate compound (UV-curing) and two epoxy compounds (thermal-curing), respectively. The measurement accuracy of the proposed approach is corroborated by an additional measurement under the identical curing conditions. The results can be used to predict the cure-induced residual stresses of dual curable adhesives.