This paper describes the 24-layer stratosphere/troposphere general circulation model (24-L ST-GCM) developed by the Climate Research Group of the University of Illinois at Urbana-Champaign (UIUC). The model's dynamical and physical processes, similar to those of its ancestors, are briefly described. The newly implemented parameterizations of longwave and solar radiation, cloud-radiation interaction, subgrid-scale orographic gravity wave drag, and aerosol radiative forcing are described in more detail. Sensitivity tests using simplified one-dimensional column models and the 24-L ST-GCM are carried out to evaluate these implementations. A 15-year simulation with prescribed climatological sea-surface temperatures and sea-ice extents has been performed. To determine the model's strengths and weaknesses, the simulated results are compared with observations. The model simulates well the geographical distributions of surface-air temperature and precipitation and their seasonal variations. The simulated cloud cover and cloud radiative forcing have the observed magnitudes and latitudinal variations, except near 60 ¿S where the model underestimates the cloud cover by ~20--30%. It is found that the large-scale cloud distribution and the cloud-top altitude depend on the respective critical relative humidities for the onset of large-scale precipitation and penetrating convection. The model captures reasonably well the observed features of atmospheric temperature and zonal wind in both the stratosphere and troposphere in all seasons, with the exception of the northern stratospheric polar-night jet. The simulated Transformed-Eulerian-Mean residual circulation in the stratosphere has comparable magnitudes and distributions to those obtained by data assimilation and other general circulation models (GCMs). The two-cell Brewer-Dobson circulation is captured. The use of an orographic-type gravity wave drag parameterization is responsible for an abnormally warm northern polar stratosphere in winter, which is contrary to most other GCMs.