A new method is proposed for estimating an ultimate cyclic shear strength of cement-treated soils after along-term curing. Shaking table tests are performed on cement-treated Toyoura sand and Ube masado with different curing periods. It was found from the tests that a hyperbolic curve can be applied to approximate a relationship between the cyclic shear strength and the curing time, so that the ultimate cyclic shear strength is determined as an the asymptotic value of the hyperbora. In addition, it was clarified that an ultimate cyclic shear strength curve, which is considered as a maximum effect of cementation, is experimentally obtained.