Abstract: Under the action of earthquake, the reinforced concrete (RC) columns may subject to torsional moment, and the existence of torsion will change the failure mode of RC columns. In order to study the size effect of RC columns on pure torsional failure, a numerical model of RC columns on pure torsional failure was established by using the three-dimensional meso-scale numerical simulation method, considering the meso-heterogeneity of concrete and the bond-slip action between reinforcements and concrete. The influence of structural size, longitudinal reinforcement ratio, stirrup ratio and cross-sectional shape on torsional failure of RC columns was investigated. The results show that: the tested RC columns show brittle failure patterns, the nominal torsional strength presents obvious size effect; the longitudinal reinforcement presents little influence on the size effect; columns with square cross-section present stronger size effect than the ones with circular shape; stirrups can improve the torsional strength, while they would weaken the size effect on torsional strength. Finally, a prediction formula for nominal torsional strength in the full-size range was established by modifying Bažant’s size effect law.