The effects of side chain length and shape at the C-9 position of fluorene in poly(9,9'dihexylfluorene) (PDHF), poly(9,9'-dioetylfluorene) (PDOF), and poly(9,9'-dicyclohexylheptylfluorene) (PDHHF) on optoelectronic and photophysical properties have been investigated in terms of Forster-type energy transfer when they are blended with poly(N-vinylcarbazole) (PVK). PVK/PDHF (w/w, 90/10) shows much more improved photoluminescence and electrololuminescence properties than PVL/PDOF (90/10) and PVK/PDHHF (90/10). The rate (k(T)) and the efficiency (E-T) of energy transfer in polymer blends which are obtained from optical properties and fluorescence intensity decay reveal that they depend on the donor-acceptor distance (r), which is altered by the side chain length of fluorene, rather than the spectral overlap integral (J). PVK/PDHHF (90/10) having polyfluorene with the long and bulky cycloheptyl hexyl side chain have a long distance between donor and acceptor with which the rate and the efficiency of energy transfer decrease. However, PVK/PDHF (90/10) shows the best optoelectronic properties due to the short side chain length of PDHF.