This paper examines the influence of parametric uncertainties on the optimal sensor placement methodologies for modal analysis of a truss bridge. Four classical sensor location methodologies are employed: two based on the Fisher information matrix and two based on energy matrix rank optimization. Young’s modulus, mass density and cross sectional dimensions are considered as uncertain parameters. The independent effects and cumulative effects of these uncertain variables on the sensor configuration are studied. The optimal locations of sensors under parametric uncertainty are assessed by the use of three different criteria. Furthermore, the robustness of this configuration is investigated for different levels of signal-to-noise ratio. The numerical results show the parametric uncertainties have significant influence on the optimal sensor configuration of a truss bridge.