We study the electronic structure of cobalt pnictides: BaT2P2 and BaT2As2 (T=Co, Rh, Ir) by the density functional theory within generalized gradient approximation, and find that the ferromagnetisms in BaCo2As2 and BaCo2P2 are due mostly to the high density of states (Van-Hove singularity) at Fermi level, which induce Stoner ferromagnetism. In these compounds, from Co-3d to Rh-4d, then to Ir-5d, the d-d bonding and the d-p bonding in the TX4(X=P, As) layers are strengthened. As a result, the antibonding d-d states are pushed away from the Fermi level, and the ferromagnetisms are suppressed in Rh and Ir compounds. The evolutions and the detailed electronic structures of these compounds are studied and compared, and spin orbital coupling interaction is negligible.