In the natural course of preeclampsia, proteinuria usually begins to improve within a few days after delivery. In heavy proteinuria, complete resolution may take weeks to months. In this case, hypertension and proteinuria persisted for 3 months after delivery. In addition, an uncommon complication—pericardial effusion—also occurred. This 34-year-old woman delivered a baby two months ago, and she had type 2 diabetes mellitus for two years. During the second trimester, heavy proteinuria with hypertension was noted; therefore, preeclampsia was diagnosed. However, tachycardia with four-limbs edema persisted after delivery, and a large amount of pericardial effusion was detected by transthoracic echocardiography. The renal biopsy revealed diabetic nephropathy superimposed with glomerular injury of preeclampsia. Preeclampsia may be complicated by severe pericardial effusion and fluid overload, and its symptoms may persist after delivery. Glycemic and intensive blood pressure control could improve the urine protein/creatinine ratio.