||The advantage of sexual reproduction over asexual reproduction, despite its high cost, is genetic mixing. Beside obligate sexual and obligate asexual reproduction, there are some species that reproduce facultatively, which can proceed with both types of reproduction. Facultative sex is suggested to possess most of the advantages of sexual reproduction by genetic mixing, while reducing the cost-of-sex, which may be more advantageous than obligate sexual reproduction. In this thesis, we develop a facultative sex model, where indivliduals can reproduce sexually or asexually with a certain frequency, σs. We determine the condition for σs to be optimal, for a given value of cost-of-sex, c, where individuals reproducing sexually with frequency σs can outcompete other individuals. We proceed with numerical computation, and determine how optimal values of σs change with c.