BACKGROUND Vitamin D has been well known for its function in maintaining calcium and phosphorus homeostasis and promoting bone mineralization. There is some evidence that in addition to sex steroid hormones; the classic regulators of human reproduction, Vitamin D also modulates reproductive process in women and men. OBJECTIVE The aim of the study is to characterize the relationship between Serum 25- hydroxyl Vitamin D levels and clinical pregnancy rates in women who undergo IVF treatment. METHODS It was a prospective cohort study which was conduct at the IVF Centre of the National Hospital, Abuja. One hundred and fifty (150) eligible subfertile women between 18 and 40years of age with regular menstrual cycle attending the IVF clinic for infertility management were recruited into this study. They were on long protocol for ovulation induction. Their serum sample for 25(OH)D was collected at the time of cycle preparation. The outcome is clinical pregnancy rate, as defined by sonographic presence of gestational sac at 6weeks gestation. Statistical analysis was performed using computer software SPSS version 21.0. Chi square test, ANOVA and H Kruscal Wallis Test were used to assess the relationship between the outcome variables and each independent variable. Factors significantly associated with the outcomes at univariate level were entered into a logistic regression model to adjust for confounding and assess the role of serum vitamin D and IVF outcome. A probability of p<0.05 was considered for tests of significance. The results were presented in frequency tables and charts.