Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly(9,9'-di-n-octylfluorene-ran-N,N'-bis(4-n-butylphenyl)-N,N'-diphenyl-1,4-benzenediamine) (F8-PDA) and poly(9,9'-di-n-octylfluorene-ran-N,N'-bis(4-t-butylphenyl)-N,N'-di- pheny1-9,10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 X 10(8) s(-1). On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet-triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is discussed in terms of the aromaticity of the amine units. On the basis of these analyses, we discuss a strategy for further improvement in the efficiency of fluorene-amine copolymer based light-emitting diodes.