This study was designed as cross-sectional on 83 healthy women aged 15 - 45 years old randomly selected from Ha Noi and Ha Nam. Study objective was to determine serum level of beta - estradiol in three phases of menstrual cycle and some associated factors. The results showed that 13 - estradiol level was the lowest in the follicular phase (51,0 +/- 19,6 pg/ml), attained the highest in the ovulation phase (224,7 +/- 42,8 pg/ml) and then decreased in the luteal phase (124,6 +/- 23,4 pg/ml). Estradiol serum level in the ovulation phase of women with no children was higher compared to women with children (p 0,05). The associations between serum level of estradiol with age, weight, height, BMI and residence were not found (p 0,05).