The possibility of group VA mono-doping and B-VA (VA=N, P, As, Sb) co-doping impurities being effective shallow donors in diamond is explored through first principles calculations. It is found that the solubility of VA atoms in diamond can be improved by the introduction of B atom. The defect formation energy is minimum and the solubility is highest for the B-N co-doping system. The configurations of adjacent impurity atoms are easier to form in B-N co-doping diamond, i.e. NBN and BNN. The NBN impurity can introduce a shallow donor level (0.55 eV) in diamond, while the enhanced contribution of N atoms to the impurity state leads to the creation of deep donor level in diamond for the BNN impurity. The current paper clearly demonstrates that the NBN complex is a possible candidate for the preparation of n-type diamond from the theoretical aspect, providing a solid theoretical basis for experimental studies on the B and N co-doping scheme.