This paper investigates the behaviour of composite box-girder bridges subject to environmental thermal effects such as solar radiation, air temperature and wind speed. It is based on the statistical analysis of measured temperature data which were recorded hourly in a newly constructed composite box-girder bridge for about 20 months continuously. Major thermal loading parameters that characterize the temperature profile are defined, and the seasonal behaviour of the parameters is described in detail. The experimental results are compared to the specifications suggested in the design codes for thermal loads. In addition, a series of heat transfer analyses using the two-dimensional finite element method is conducted in order to generalize the results to bridges with different cross-sections. The findings of this study show that the design temperature distribution with uniform differential between the concrete slab and the steel girder is unsuitable for representing the thermal effects in design purpose.