We studied the oligomerization of Alzheimer amyloid beta peptide (A beta) using a replica exchange molecular dynamics (REMD) simulation. The simulation was performed with A beta(10-35) dimers, trimers, and tetramers. Extensive REMD simulations illustrated several possible oligomer conformations. As the size of the oligomer increased from a dimer to a tetramer, the number of possible configurations was reduced. We identified all the possible conformations for each oligomer and characterized their temperature dependence. It was found that the detailed structures of the oligomers, which may act as folding intermediates, are highly sensitive to the parameters of the simulation environment such as temperature and concentration. Structural diversities of A beta oligomers suggest multiple pathways of the aggregation process.