Effective chain extenders for linear polyesters such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) were sought among some nitrogen-containing bisheterocycles, which were reactive with hydroxyl terminals of the polyesters to form addition linkages. Tested were such bis cyclic-imino-esters as bis[5(4H)-oxazolone]s (OXZ) and bis(4H-3,1-benzoxazin-4-one)s (BNZ), among which BNZ, especially 2,2′-bis(4H-3,1-benzoxazin-4-one) (BNZ-A-1) has been proved to be a very effective chain extender for both PET and PBT. Starting from PET having intrinsic viscosity ([η]) of 0.52 and carboxyl content (CV) of 10 eq/106g, addition of 1 mol % BNZ-A-1 resulted in PET having [η] of 1.11 and CV of 13 eq/106g at a polyester melt temperature under atmospheric nitrogen within a few minutes. On the other hand, OXZ which were less thermally stable than BNZ could not act as good chain extenders for PET at a PET melt temperature. But they were effective in the case of PBT, which melted at a lower temperature than PET.