Selection of sex at or before conception is perhaps the most desired biotechnology of all time because many economic traits are related with sex. Sex selection by manipulating the conditions of sperm-oocytes fertilization would be one of the most practicable and widely used methods. This review summarized the progress in the effects of fertilization conditions on mammalian sex. Four main factors which possibly affected sex were described such as arginine concentration in female animal vagina, glucose in the uterus, maturational status of the oocyte at the time they encounter sperm as well as the hormone levels of both parents at the time of conception.