We designed and synthesized new blue emitting Ir(III) complexes, tBuCN-FIrmMes and tBuCN-FIrdMes, with the newly introduced ancillary ligands, 1-mesitylbutane-1,3-dione (mMes) and 1,3-dimesitylpropane-1,3-dione (dMes), respectively. The alteration of ancillary ligand with only one mesityl group resulted in excellent photophysical properties for tBuCN-FIrmMes such as the highest PLQY (96%), high Kr (8.73×10(5) s(-1)) and very low Knr (0.36×10(5) s(-1)) leading to the superior device performance compared with the PhOLEDs of tBuCN-FIrdMes. For instance, the PhOLED of tBuCN-FIrmMes showed EQEs of 22.4 % (max.) at 1000 cd m(-2), 19.8 % at 10 000 cd m(-2), and 16.5 % at 20 000 cd m(-2), that are higher compared with the PhOLED of tBuCN-FIrdMes; the EQEs are 17.8% (max.) at 1000 cd m(-2) and 14.9 % at 10 000 cd m(-2).