Neonatal convulsion is a very alarming manifestation of underlying sinister problem. As an important cause, neonatal hypocalcemia usually occurs soon after birth and reflects abnormal maternal calcium regulation. We report an unusual case of late neonatal hypocalcemia presented with intractable seizures after two-month-old. Further investigations revealed maternal hyperparathyroidism secondary to parathyroid adenoma, which was missed during antenatal checkup. High index of suspicion is necessary in early detection and prompt treatment of neonatal hypocalcemia. Screening of maternal calcium level is mandatory in unexplained late neonatal hypocalcemia as mother with primary hyperparathyroidism could be asymptomatic and first manifest as late-onset neonatal convulsion. Early detection and appropriate intervention could avoid serious maternal and fetal morbidity.