This paper deals with the damping properties of a viscoelastic three-layered cantilever with partial delamination. The expression for the loss factor of this cantilever is presented as the ratio of the dissipation energy to the strain energy. In analysis, the loss factors due to shearing and bending are determined independently, and the total loss factor is derived from their collective sum. According to this analysis, the effects of the delamination length, delamination position and thickness of each layer on the loss factor corresponding to each vibration mode are investigated. As a result, it is clarified that delamination makes the magnitude of the loss factor larger than in the case of no delamination, and that the value of the loss factor increases as the delamination length becomes large. In experiments, it was ascertained that the experimental values of the loss factor were in good agreement with the theoretical ones.