Study on the patterns of successive BN pair substitution in C-50 fullerene and the chemical and electronic properties of these substitutionally doped heterofullerenes has been carried out with semiempirical (AM I and MNDO) and density functional (B3LYP/3-21G) theories. The BN units prefer to stay together following "single bond", "hexagon filling", and "continuity and equatorial belt" rules. The driving force governing the stabilities of these BN-doped fullerenes is the strain of the cage. Compared with C-50, the redox activity of C50-2x(BN)(x), (x = 1-15) isomers decreases and becomes weaker by increasing the number of BN units, while the aromaticity of the C50-2x(BN)(x), derivatives decreases and is independent of the number of BN units but related to the substitution positions. The main infrared absorptions are similar for all the C50-2x(BN)(x) isomers and the infrared spectrum becomes simpler and stronger with increasing the number of BN groups.