In this paper the variability of modal properties caused by temperature effects is assessed in order to adjust modal data used for nondestructive damage detection in structures. First, an experiment to estimate temperature effects on modal properties is performed. Pre-damage and post-damage dynamic modal data from a model plate-girder bridge are described. The relationship between temperature and natural frequencies is analyzed and a set of empirical frequency-correction formulas are obtained for the test structure. Next, a frequency-based damage-detection method is utilized to locate and estimate severity of damage in the test structure for which a set of pre-damage and post-damage frequencies were measured at different temperature conditions. A theory of the frequency-based damage-detection algorithm that related to beam-type structures is outlined. The measured frequencies are adjusted by the frequency-correction formulas and fed into the damage-detection scheme that locates damage and estimates severity of damage in the test structure. Results of the analysis indicates that the temperature correction scheme works for the accurate damage localization and severity estimation in the test structure.