Solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer (FHQ, FPQ) to a fluorescein (Fl)-labeled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO), optical properties of the polymers and the probe dye were substantially modified. And the FRET-induced Fl emission was measured by directly exciting the polymer within the complex, polymer/ssDNA-Fl. The FRET signal was successfully modulated with changing the DMSO content. In the case of FHQ, the FRET-induced Fl emission was seriously quenched in phosphate buffer solution (PBS), while a salient FRET signal was observed in a 80 vol% DMSO/PBS mixture (36.8 time higher than that in PBS). The FPQ-sensitized FRET signal was also 3.8-fold amplified by the presence of DMSO. That result is from the decrease of hydrophobic interactions between the polymer and ssDNA-Fl, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular separation. The gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern- Volmer quenching constants (3.3-4.2 × 10 6 M-1 in PBS, 0.56-1.1 x 106 M-1 in 80 vol% DMSO) in PBS/DMSO mixtures. The substantially reduced PL quenching would amplify the resulting FRET Fl signal. This approach suggests a simple way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated polymer-based FRET bioassays.