Objective To observe the value of MRI in evaluation on the morphology and function of iliococcygeal muscles in fertile and nulliparous women.Methods Totally 50 healthy fertile women (fertile group,further divided into cesarean section subgroup and spontaneous delivery subgroup according to the mode of delivery) and 17 nulliparous healthy women (nulliparous group) underwent MR scanning in both natural and increased abdominal pressure state.Iliococcygeus thickness (ICT),coronal iliococcygeal angle (cICA) and sagittal iliococcygeal angle (sICA) of different states were measured and compared between the groups.Results In the natural state,the right and bilateral average sICA in the fertile group were larger than those in nulliparous group (both P＜0.05),while no statistical difference of ICT,right,left and bilateral average cICA and left sICA were found between two groups (all P＞0.05);the bilateral average sICA in spontaneous delivery subgroup was larger than that in cesarean section subgroup (P＜0.05).In increased abdominal pressure state,left,right sICA and bilateral average sICA in fertile group were larger than those in nulliparous group (all P＜0.05),while there was no statistical difference of ICT and cICA between two groups (all P＞0.05);no statistical difference of ICT,cICA nor sICA was found between spontaneous delivery subgroup and cesarean section subgroup (all P＞0.05).Conclusion MRI can accurately evaluate morphological and functional changes of iliococcygeal muscle in females.