Objectives: Placental mesenchymal dysplasia (PMD) is different from the hydatidiform mole histologically. Rare disease of PMD is often a mistaken diagnosis because of the similar multicystic sonographic findings. The aim of the present case report was to demonstrate characteristics of PMD. Case presentation: Pregnant woman was referred at 20 weeks of gestation because of suspicion of partial hydatidiform mole. Ultrasound revealed normal fetal growth without morphological anomaly, enlarged placenta attached to the anterior wall included large multiple cysts and dilated chorionic vessels. Maternal serum human chorionic gonadotropin level was 25,961 IU/L. Based on these results, diagnosis of PMD was made. After genetic counseling, patient elected to continue the pregnancy. With advancing gestation, placenta continued to be large and hydropic, but the fetal growth was within normal. Due to complication of pre-eclampsia at 37 weeks of gestation, she delivered a healthy female infant (2289 gm) and enlarged placenta weighed 1838 gm. Conclusion: Placental mesenchymal dysplasia that often complicates both maternal and fetal several abnormalities is one of the important perinatal disorders which required accurate ultrasound diagnosis and appropriate genetic counseling from the early gestation.