Abstract: Body mass or weight is a crucial biological parameter for an organism. It is influenced by development, reproduction, physiology and evolution. Therefore, mass estimates for fossil species are important for many kinds of analyses. In this project, eighteen bivariate regression analyses of different measurements of the appendicular skeleton plotted against body weight in a data set of 422 individual birds, representing 229 species in 21 orders, revealed high correlations between several skeletal parameters and body mass. R-squared values of eighteen bivariate equations are ranged from 0.50 for tibiotarsal length, indicating a relative poor fit, to 0.91 for humeral diameter. To test the 18 equations empirically, they were used to predict the body weight of an additional 64 extant bird specimens, and the accuracies of various equations were compared. This predictive test showed that three parameters are generally most accurate as predictors of body mass: humerus length, ulna diameter, and tibiotarsal diameter. However, the humeral length and ulna diameter tended to give accurate results for particularly songbirds, raptors and climbing birds. The tibiotarsal diameter tended to give accurate results for terrestrial birds, such like chicken and doves. It is probable that humerus length and ulna diameters are the more accurate parameter for arboreal taxa, while tibiotarsal diameter is more accurate for terrestrial ones. Closer examination of the results showed that different measurements correlated best with body mass in different avian orders. This variation appeared to result from differences in habitat and functional morphology across the avian orders represented in the data set. The weights of some Chinese Mesozoic fossil birds were estimated using the equations generated for humeral length and tibiotarsal diameter, because ulnar diameter was frequently difficult to measure. Humeral length and tibiotarsal diameter yielded dramatically different mass estimates for some taxa, with estimates based on humerus length generally being lower. The result shows that these Early Cretaceous birds experienced a significant diversification in body weight during evolutionary process.