Efficient deep blue-emitting poly[9,9-bis(2-ethylhexyl)fluorene] derivatives were synthesized via incorporating 2,8-diyl-dibenzothiophene-S,S-dioxide (FSO) moiety into polyfluorene backbone. The photoluminescence (PL) spectra exhibit excellent thermal stability and PL quantum efficiencies distinctly improved upon thermal annealing at various temperatures. Spectrally stable light-emitting diodes were obtained based on resulting copolymers at various driving voltages and operation times. The best device performances were achieved based on copolymer PF-FSO10, which shows a maximum external quantum efficiency of 1.78%, and 2.27% upon thermal annealing in the air, and as high as 3.52% by inserting an additional electron injection layer poly(9,9-bis(6′-(bromilamino)hexyl))fluorene (PFNBr) between the polymer-cathode interface. The stable electroluminescence spectra with CIE coordinates of (0.16, 0.08) and superior device performances indicates that the copolymers could be a promising candidate of deep blue emitters in full-color displays.