Abstract： Objective To demonstrate the value of eombined application of prenatal ultrasonography with fetal magnetic resonance imaging(MRI) in the diagnosis of monochorionic muhifetal realformations. Methods Fourteen cases of muhifetal malformations,detected by prenatal ultrasonography,received MRI within 48 h afterwards.All diagnosis were confirmed after delivery or mid-term termination.All imaging results of the 14 cases were retrospectively reviewed. Results Among the 14 cases,there were 7 acardias,5 Conjoined twins and 2 demise of multifetuses.Comparing ultrasound with MRI,we found that:(1)In cases with acardia and demise of multifetusea,ultrasound could diagnose correctly and be an important tool for follow-up,while MRI could demonstrate organs and structures of the acardiac recipient more clearly and detect the secondary changes of brain in the donor and survived fetus.(2)In Conjoined twins,ultrasound was superior to MRI in demonstrating the structure and function of cardiovascular system : and equivalent to MRI in identifying stomach,kidney,bladder and limbs;but inferior to MRI in identifying esophagus,lung,liver and intestinal,especially in the brain. And MRI could demonstrate two fetuses and the relationship between them in COnjoined twins simultaneously. Conclusions Prenatal ultrasonography and MRI have their own advantages and disadvantages in diagnosing monochorionic multifetal malformations.But the combination of prenatal ultrasonography and fetal MRI may be more valuable.