Main-chain polybenzoxazines containing amide linkages were successfully prepared in one pot. Three different polymers were synthesized by reacting 3,4-dihydrocoumarine (DHC) and paraformaldehyde with 1,3-diaminopropane or 1,6-diaminohexane or Jeffamine ED-900. The one-pot reaction proceeded through the combination of the ring-opening of DHC with amines, and subsequent Mannich and ring-closure reactions occurring in a cascading manner. The obtained polymer from Jeffamine exhibited good film-forming properties, and free-standing flexible films were easily solvent- casted on Teflon plates. All polymeric precursors were characterized by spectroscopic analysis, and their curing behavior and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).