Dietary unsaturated fatty acids (UFAs) have been implicated in several reproductive processes through a variety of mechanisms. Some of these effects were altered by the type of FA fed. The polyunsaturated FA of the n-6 and n-3 families seems to have the most remarkable effects on reproductive responses of ruminants. This study examined the effects of supplementation of rumen bypass fats, n-3 and n-6 poly unsaturated fatty acids (PUFAs), on ovarian follicle development and steroidogenic parameters and also the expression of one of oocyte-derived growth factors namely bone morphogenetic protein 15 in ovine oocyte. Bone Morphogenetic Protein 15 is critical for ongoing development to ovulation, most likely by regulating the proliferative and differentiative functions of adjacent follicular cells. Thirty 2-4 year old ewes were divided into three groups and were fed randomly a control diet (n=10) or supplemented with fatty acids, n-3 (n=10) or n-6 PUFAs (n=10) over a 7-week period. Oestrus cycles were synchronized using progesterone releasing implant (controlled internal drug releasing (CIDR) device). Beginning at estrus, ovarian follicular population was monitored daily via transrectal ultrasonography, during this period the number of ?2 mm follicles quantified. Blood samples were collected every 3 day, during synchronized oestrus cycle. At 13 days following behavior estrus ewes were slaughtered and all visible follicles were individually punctured and follicular fluid was released in a Petri dish. Cumulus oocyte complexes were isolated and divided into two groups split according to follicle diameter (2–4 and ? 5 mm). RNA isolated from oocytes of each follicle category of each sheep. Relative expression of Bone Morphogenetic Protein 15 transcript in oocytes was determined by Quantitative Real-time PCR. Feeding n-6 PUFA increased the number of medium-size (2-3 mm) and large (? 4 mm) follicles. There was a tendency for a higher number of follicular waves in n-6 PUFA group compared with the other groups. Feeding n-3 PUFA had no effect on follicular waves or number of medium-size and large follicles in this group. lasma cholesterol, High Density Lipoprotein and progesterone were all higher in ewes fed a diet supplemented with n-6 PUFA than in ewes not received this supplement. Diet had no effect on follicular fluid progesterone. The Bone Morphogenetic Protein 15 transcript expression in oocytes that were collected from the 2-4 mm follicles was higher in n-3 and n-6 PUFAs group than control. But n-3 and n-6 PUFAs had no effect on expression of Bone Morphogenetic Protein 15 in oocytes that were collected from ? 5 mm follicles.