A novel asymmetric diamine, 2-benzimidazolyl-4,4’-oxydianiline (6), containing a bulky benzimidazole substituent and flexible ether linkage was synthesized successfully. Novel polyimides PI 8a~c were prepared by two-step polycondensation of diamine (6) and three commercially available aromatic dianhydrides. PI 8a~c showed excellent thermal stability with glass transition temperatures of 277 to 342 ℃ and the 5% weight loss temperatures of 528-573 ℃ in nitrogen. PI 8a~c with high molecular weights also exhibited good mechanical properties in both their dry and phosphoric acid doped states. They demonstrated better oxidative stability in Fenton test than m-PBI. Their proton conductivity ranged from 0.017 to 0.043 Scm^-1 at 160 ℃ in anhydrous conditions with phosphoric acid (PA) uptakes from 89 to 116 %. PI 8b containing hydrophobic trifluoromethyl groups had the highest proton conductivity of 0.043 Scm^-1 with the lowest PA uptake (89%). In the fuel cell test performed with H₂/O₂ under anhydrous conditions at 160 ℃, the peak power density of PI 8b with PA uptake of 89 % was 567 mWcm^-2, which was higher than that of m-PBI (423 mWcm^-2) with PA uptake of 221 %. The higher proton conductivity and peak power density of PI 8b were attributed to the hydrophobic trifluoromethyl groups that might facilitate the formation of the ionic clusters or channels at lower PA uptakes.