Polycoordinate planar B compounds BX(n) (X=B, Al, C, N and Si; n=3-8) are optimized at B3LYP/6-311++G (3df,p) theoretical level. For X=B, center B atom can coordinate three to eight atoms, while for X=Al, C, Si, and N, it can only coordinate three to five atoms. The natural bond orbital analysis shows that the center B atom does not violate the octet rule, though the numbers of coordinated atom even reach 8. According to molecular orbital analysis and nucleus independent chemical shift value calculation, it seems that these polycoordinate planar B compounds BX(n) (X=B, Al, C, N, and Si; n=3-8) hold twofold (alpha and pi) aromatic, which play an important role in their stability and keeping all atoms in one plane.