Energy barriers were formed in PIC Ⅰ dye doped with azaPIC Ⅰ changing the physical sizes of PIC Ⅰ J aggregates. Dephasing processes of coherent Frenkel excitons were studied using accumulated photon echo with double modulation and heterodyne detection technique. The lengthening of dephasing time T 2, from 60 ps to 224 ps, were observed with increasing the molar fractions of azaPIC Ⅰ. It is contrary to the shortening of T 2 in the mixed aggregates with traps. The coherence lengths of excitons in the mixed aggregates were investigated theoretically and experimentally.