The stiﬀness of the elastic ﬁn for underwater propulsion may need to change concerning its bending speed. In order to realize the bending speed dependence of stiﬀness in relation to the structure, we have developed an anisotropic viscoelastic ﬁn containing ﬁber composite dilatant ﬂuid. For the bending test of the ﬁn to examine its stiﬀness, the top of the ﬁn is ﬁxed, while the base of the ﬁn is rotated. The bending resistance force, as the apparent stiﬀness of the ﬁn, was measured by the load cell connected to the top of the ﬁn. Bending resistance of the ﬁn containing dilatant ﬂuid increased as the average angular velocity of bending increased. Furthermore, bending resistance of the ﬁn increased according to the existence of ﬁber, especially the ﬁn along the longitudinal direction of the ﬁber. This developed ﬁn is a highly functional structure, which can adapt to the demands of changing optimum stiﬀness with regard to diﬀerent directions of deformation speed.