The limiting of alkali content in concrete is effective on mitigation of ASR and total alkali content in concrete are limited under 3kg/m3 in Na2Oeq in Japan. Flowever, ASR was evidenced in some structures in which alkali content was limited under 3kg/m3. One of the reasons why ASR was caused in limited alkali condition is considered as a pessimum phenomenon. This study focuses on the mitigating effect of fly ash on the ASR-related expansion of mortar using the reactive aggregate at the pessimum proportion. In this study, 3 types of andcsites were used as reactive aggregate. These aggregates had the pessimum proportion of 30% by mass at ccmcnt alkali of 1.20%. Thus, mitigating effect of fly ash was evaluated by mortar expansion test in which the reactive aggregate was mixed at the pessimum proportion and at 100% of total aggregate. As a result, the mitigating effect of fly ash with the replacement level of 20 vol.% became much less at the pessimum proportion than at 100% of reactive aggregate ratio. High replacement level of fly ash is expected to be demanded in the case of the pessimum condition.