We report the supermolecular interactions, electronic absorption spectra, and dynamic second-order nonlinear optical polarizabilities beta of pi-stacking (p-) and hydrogen-bonded (h-) dimers and a trimer in a 2-methyl-4-nitroaniline (MNA) crystal, using the TDB3LYP/3-21G+ coupled with the SOS methods. The calculated results show that the supermolecular interactions and second-order optical polarizabilities are smaller for dimers (p- and h-dimers) than the trimer in MNA crystal, which originates from size effect, and the h-dimer has smaller supermolecular interaction and larger second-order nonlinear optical response compared with that of the p-dimer, which originates from site effect in MNA crystal. The lowest absorption peak comes from the pi-pi* charge transfers within a MNA molecule for h-dimer, and between the two MNA molecules for the p-dimer in a MNA crystal. The calculated polarizability norm parallel tobetaparallel to of trimer consisting of h- and p-arrangements is 42.0 x 10(-30) cm(5) esu(-1), and the macroscopy susceptibility norm parallel tochiparallel to is 6.15 x 10(-7) esu as compared with the experimental value parallel tochiparallel to of 9.06 x 10(-7) esu in MNA crystal. The slipped cofacial arrangements of dimer (h-dimer) have larger contributions to the electronic absorption spectrum and nonlinear optical response of MNA crystal.