The full non-rigid (f.NRG) molecules group theory is seen to be used advantageously to study the internal dynamics of such molecules. In this work, a computational method is applied to study the f.NRG of cis- and trans- dimethyl diborane, trimethyl diborane and tetramethyl diborane with different point groups. We considered the point groups of them and computed the other, conjugacy classes and the corresponding symmetry for each permutation. Also the irreducible character tables of them are calculated. Key words: Non-rigid group, character table, cis- and trans-dimethyl diborane, trimethyl diborane, tetramethyl diborane.