Poly (amido-amine) (PAMAM) as one of the earliest synthesized dendrimers is now widely used in biomedicine, host-guest chemistry and so on. PAMAM dendrimers from generation 2 to 6 were studied by all-atom molecular dynamic simulations using different force fields and different coarse-graining schemes. The DREIDING force field is found to provide the best accuracy in predicting the structures and properties of PAMAM dendritic molecules. Simulation with the coarse-grained approach decreases the considerable CPU-time cost while keeping commensurate accuracy with all-atom simulations under the same simulation condition. The radius of gyration, solvent accessible surface area, solvent accessible volume and fractal dimension are calculated as functions of generation. Results show that PAMAM dendrimers have a fractal structure with dimensions between 2.0 and 3.0. The ample external terminal groups and internal voids make the PAMAM dendrimer an excellent candidate for drug carriers. The terminal groups of PAMAM dendrimers are sufficiently flexible to interpenetrate almost the whole molecule. The simulation results on different coarse-grained models are compared; the coarse-graining scheme has significant influence on the simulation accuracy. (C) 2010 Elsevier B.V. All rights reserved.