Potential-energy surfaces for unimolecular decomposition of B(N-3)(3) have been studied to understand the possible mechanism for BN generation. The decomposition of B(N-3)(3) takes place on either the singlet and triplet surface, and both processes are high exothermic and obey sequential mechanisms. For the singlet reaction, the rate-determining step corresponds to cleavage of the first azide bond and linear (NBNN)-N-1 instead of (BN)-B-1 was suggested as the dominant product at room temperature. For the triplet surface, a fragment process from (BN7)-B-3 to (BN5)-B-3 is predicted to be the rate-determining step; once this barrier is counteracted, the subsequent decomposition processes could easily occur to form final product (BN)-B-3. In addition, the possible mechanism for generating BN film via B(N-3)(3) was discussed based on MC-SCF calculation results. These findings might be helpful in understanding the controllable decomposition of B(N-3)(3) as well as its application in generating BN films.