Maternal vitamin D deficiency is one of the major risk factors for neonatal vitamin D deficiency followed by neonatal hypocalcaemia. The aim of this study is to determine the relationship between delayed neonatal hypocalcaemia and maternal vitamin D deficiency. This is a descriptive cross-sectional study. Target population of this study included all term and preterm neonates with delayed hypocalcaemia (after the first 72 hours of birth) admitted to Ali-Asghar Hospital. The sample size was 100 neonates included in the study. Demographic, clinical and paraclinical data including Ca, P, PTH and level of maternal and neonatal vitamin D were recorded according to patients records. 67 neonates (67%) were term and 33(33%) were preterm neonates. The mean of serum calcium in neonates was 6.49± 0.68mg/dL (in the range of 4.3-7.8 mg/dL). 85% of neonates and 74% of mothers had vitamin D deficiency. 100% of neonates born to mothers with vitamin D deficiency were hypocalcaemia. A statistically significant difference was seen between the mean values of serum Ca (6.67 in term vs. 6.12 in preterm neonates) and vitamin D in term and preterm neonates was 16.34 vs. 20.18 (P= 0.0001 and P=0.01 respectively). Also, a significant correlation was seen between maternal and neonatal level of vitamin D (P=0.0001, r=0.789). With regard to the socio-cultural status in Iran besides womenʹs clothing style and nutritional deficiencies before and during pregnancy, health authorities and policy makers are responsible to focus their serious attention on hypocalcaemia and hypovitaminosis D in neonates.