Protein assumes specified conformations from their chemical compositions or sequences to develop biological activity and functional specificity.The corresponding three-dimensional structures are a consequence of inter-and intra-molecular interactions.It was shown that there exists a universal geometric factor of protein conformations as a consequence of energy minimization.The factor defined as the dimensionless ratio (V/A〈r〉) of the van der Waals volume V to the surface area A and average atomic radius 〈r〉 of the folded structures, for real and well-designed proteins is universal and equal to 0.491±0.005.We propose packing models for protein structures to satisfy the universal ratio.