Novel phosphorescent emitters, FIr4mpic and FIr4tpic were designed and prepared synthetically to improve the chromaticity and emission efficiency of FIrpic, one of the main blue phosphor. Apparently, methyl and tert-butyl on 4-position of picolinic acid of FIrpic were installed attaining FIr4mpic and FIr4tpic, respectively. With these new ancillary ligand-tuned cyclometalated iridium(III) complexes, phosphorescent organic light-emitting diodes (PHOLEDs) devices were constructed. The ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra of FIr4mpic and FIr4tpic were similar to those of FIrpic; however, in terms of their electroluminescent properties, higher luminance was observed with FIr4mpic device. FIr4mpic and FIr4tpic exhibited deeper blue light emission than that of FIrpic. Once optimized, a maximum external quantum efficiency (EQE) of 18.77% and a maximum current efficiency (CE) of 36.17 cd/A were achieved with the FIr4mpic-based PHOLED. Along with the experimental information we also investigated the electronic properties by means of density functional theory (DFT) calculations being keen to explain the improved quantum yield of FIr4mpic over the other emitters.