A comparative study on the folding of multiple three-alpha-helix bundle proteins including alpha D-3, alpha W-3, and the B domain of protein A (BdpA) is presented. The use of integrated-tempering-sampling molecular dynamics simulations achieves reversible folding and unfolding events in individual short trajectories, which thus provides an efficient approach to sufficiently sample the configuration space of protein and delineate the folding pathway of a-helix bundle. The detailed free energy landscape analyses indicate that the folding mechanism of alpha-helix bundle is not uniform but sequence dependent. A simple model is then proposed to predict folding mechanism of a-helix bundle on the basis of amino acid composition: alpha-helical proteins containing higher percentage of hydrophobic residues than charged ones fold via nucleation- condensation mechanism (e.g., alpha D-3 and BdpA) whereas proteins having opposite tendency in amino acid composition more likely fold via the framework mechanism (e.g., alpha W-3). The model is tested on various alpha-helix bundle proteins, and the predicted mechanism is similar to the most approved one for each protein. In addition, the common features in the folding pathway of alpha-helix bundle protein are also deduced. In summary, the present study provides comprehensive, atomic-level picture of the folding of alpha-helix bundle proteins.