The aim of the study was to evaluate the efficacy of melatonin administration on oocyte quality in women underwent in vitro fertilization (IVF) cycles. Eighty-five women undergoing IVF cycles were randomized in two groups during IVF-embryo transfer (ET) procedure, 40 women with melatonin treatment (A) and 45 women without melatonin treatment (B). Primary endpoint was the number of morphologically mature oocytes retrieved (MII oocytes). Secondary endpoints were fertilization rate per number of mature oocytes, embryo quality and pregnancy rate. There were no differences between two groups according to age, and peak estradiol levels. The mean number of oocytes (15.33 vs. 14.27) and the mean number of mature oocytes did not differ between the two groups (12.63 vs. 10.94), whereas the percentage of mature oocytes (M2/oocytes retrieved) was significantly different in melatonin-treated group (p < 0.05). Fertilization rate (72.75 vs. 71.16) did not differ between the two groups. The mean number of class 1 embryos resulted higher in the group A (3.28 vs. 2.53) (p < 0.05). Clinical pregnancy rate was in tendency higher in the group treated with melatonin, although the differences did not reach statistical significance. Melatonin is likely to improve oocyte and embryo quality in women undergoing IVF or intracytoplasmic sperm insemination (ICSI).