Objective To evaluate the volume change of rat root following direct pulp capping with mineral triozide aggregate (MTA) and calcium hydroxide-based paste (Vitapex).Methods Sixty-four female,1-month-old Wistar rats were randomly divided into 4 groups,MTA group,Vitapex group,model group and control group,16 rats in each group.The right maxillary first molar was taken as experimental tooth and control tooth.The exposed pulp was capped separately with MTA and calcium hydroxide after the pulp had been exposed mechanically,while nothing done to the control group.Using micro-CT and three-dimensional reconstruction techniques,the volume change of rat root was evaluated at 1,2,4 weeks and 6 weeks after direct pulp capping.The data were analyzed by an way ANOVA analysis followed by a LSD-t test.Results The root volume in the MTA group [(1.08 ± 0.07),(1.32 ± 0.18) mm3] was significantly smaller than that in the Vitapex group[(1.28 ±0.16),(1.59 ±0.18) mm3] at 2 and 4 weeks after operation(P ＜0.05).At the sixth week,there was no significant difference between the MTA group [(1.36 ± 0.03) mm3] and the Vitapex group [(1.61 ± 0.31) mm3] (P ＞ 0.05).The root volume in the MTA group and Vitapex group was significantly larger than that in the model group [(0.87 ± 0.09),(1.01±0.17),(1.02 ± 0.25) mm3] from the second to sixth week after operation(P ＜ 0.05).Conclusions MTA and Vitapex can effectively promote root formation and growth and can be used as biological pulp-capping material.