Geometry mismatch in a turbine blade root, which arose in manufacturing process or caused by wearing out during service, leads to contact conditions changed in fir-tree attachments. As a result, shifting of the fundamental frequencies and redistribution of stress in the blade base possibly cause failure of the blade. A three-dimensional finite element model of a blade and its fir-tree attachments have been constructed and analyzed by taking into account contact nonlinearity in the attachments and large deformation effect of the blade. The geometry mismatch was introduced into the finite element model by defining gaps between two contact surfaces in the attachments. The influence of gap configuration and gap size on contact and fundamental frequencies was investigated. Results showed that gap configuration has significant influence on fundamental frequencies of the blade, especially on its bending modes. Gap size has little influence on the frequencies but significant influence on the contact status and thus changes stress distribution in the attachments. The results also suggest that modeling contact behavior in fir-tree attachments is necessary to obtain more accurate fundamental frequencies.