A series of fluorene diamine‐based benzoxazine monomers are prepared from the reaction of 9,9‐bis‐(4‐aminophenyl)‐fluorene with paraformaldehyde and unsubstituted or substituted phenols. Unsubstituted and substituted phenol‐based benzoxazines show different polymerization behaviors and give polymers with different thermal properties. The ortho and para positions on the phenol for benzoxazines based on phenol and m‐cresol could concurrently participate in the crosslinking reaction during thermal ring‐opening polymerization to form Mannich and methylene bridges at high temperature. The glass‐transition temperature and thermal stability of the resulting polymers are much higher than those of other substituted phenol‐based polybenzoxazines and bisphenol fluorene‐based polybenzoxazines.