To improve the stability and durability of organic EL devices, the mixed HTL layer was fabricated by co-deposition of TPD and Alq(3) and the EL cell with the mixed HTL was compared with that with the conventional pure HTL. When more than 5 wt% of Alq(3) was added in HTL, the increase in surface roughness of ITO / TPD-Alq(3) / Alq(3) by annealing was found by AFM to be reduced dramatically. In addition, the running durability was significantly improved by addition of a small amount of Alq(3) (< 10 wt %), although the EL characteristics of the cells with mixed HTLs were slightly lowered, i.e. IV and L-V curves shifted to the higher voltages.