A novel asymmetric alkylaminoborazine monomer, 2-propylamino-4,6-bis(methylamino)borazine, was synthesized for the first time, and directly polymerized to give a melt-spinnable polyborazine (PBN). This asymmetric alkylaminoborazine was synthesized by an aminolysis reaction of 2,4,6-trichloroborazine (TCB) with different amines under mild conditions. This route turns out to be much cheaper and simpler than the conventional routes. The chemical composition, structure, molecular weights and ceramic yield were investigated by EA, FTIR, NMR, GPC and TG analysis. The PBN exhibits suitable rheological property for melt-spinning, which suggests that it is a potential precursor for BN fibers.