The effect of annealing treatment conditions on the interfacial adhesion energy between electrolessplated Ni film and polyimide substrate was evaluated using a $180^{circ}$ peel test. Measured peel strength values are $26.9{pm}0.8,;22.4{pm}0.8,;21.9{pm}1.5,;23.1{pm}1.3,;16.1{pm}2.0;and;14.3{pm}1.3g/mm$ for annealing treatment times during 0, 1, 3, 5, 10, and 20 hours, respectively, at $200^{circ}C$ in ambient environment. XPS and AES analysis results on peeled surfaces clearly reveal that the peeling occurs cohesively inside polyimide. This implies a degradation of polyimide structure due to oxygen diffusion through interface between Ni and polyimide, which is also closely related to the decrease in the interfacial adhesion energy due to thermal treatment in ambient conditions.