New series of hyperbranched polyfluorenes containing triarylpyrazoline cores, PFZ10, PFZ20, PFNZ10, and PFNZ20, have been synthesized according to the "A(2) + A'(2) + B-3" Suzuki coupling method. The structures and property of the monomers and conjugated polymers were characterized by elemental analysis, gel permeation chromatography, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, and UV-visible absorption, H-1 NMR, C-13 NMR, and photoluminescence spectroscopies. All these polymers exhibited good solubility in common organic solvents and good thermal stability. The long-wavelength emission of polyfluorenes had been effectively reduced in these hyperbranched polymers. Standard polymer light emitting devices (PLEDs) from PFZ10, PFZ20, PFNZ10, and PFNZ20, with the configuration of ITO/PEDOT/polymer/TPBI/Alq(3)/Mg:Ag, exhibited good electroluminescence (EL) properties The PLED based on PFNZ10 emitted pure blue light with a low turn-on voltage of 5.3 V and a high EL efficiency of about 1.93%. (c) 2007 Wiley Periodicals, Inc.