In an effort to understand intrinsic grain boundary brittleness, computer simulation of grain boundary structures in ordered NiAl was carried out. The considered boundaries were boundaries of low reciprocal coincident site density Σ. The structural unit model was used to obtain a general picture of the grain boundary features in NiAl. Relation between the grain boundary stoichiometry and the energy was studied and it was found that the grain boundary energy increases with a higher aluminum content in the grain boundary. Thus, excess of aluminum in the grain boundary seems to make the material brittle. The grain boundaries obtained are relatively dense and grain boundary brittleness would not be expected to originate from large interstitial holes. However, the structures computed show a large spread in energy values and it is proposed that the observed brittleness is due to the appearance of high energy structures when dislocations arrive at the boundary. / Master of Science