By means of the configuration interaction the energies and the wave functions for the excited C3?u and B3?g states of the nitrogen molecule are calculated with the electronic computers TQ-16 and CROMEMCO. The set of criteria for selecting configuration function are presented. The effects of other multiplicate states on the excited C3?u and B3Πg states are excluded by using the projection operator. In the dipolar approximation the calculated value of the oscillator strength for the second positive band system of the nitrogen molecule is 0.09739，being in good agreement with experimental value. The presented method in the paper can be applied to all diatomio moleculea.