The mechanical properies of intermetallics in Ti-Al alloy system are studied systematically from the view point of valence electron by the empirical electron theory (EET). First, according to the basic thinking of the EET, space topology of valence electron structure is modelled, and the embrittlement nature of intermetallics in Ti-Al system is concisely brought to light. Second, the valence electron structure symmetry factor α, which has a good relationship to the plasticity is proposed from the model, and the embrittlement criterion is summed up. Finally, the empirical relation between valence electron structures and mechanical properties can be established.