The Temperament (mizaj) theory is backbone of Unani system of medicine. This system categorizes individuals according to the temperament. Temperament indeed occupies a very important place in Unani system of medicine and forms the basis of pathology diagnosis and treatment in this medical system. The etidal or equilibrium of this distinct temperament i.e. maintenance of constant internal environment in different individuals leads to a healthy body i.e. normal body functions. Any derangement of temperament from etadal or imbalance of normal temperament results in su-e-mizaj which causes deranged body functions i.e. diseases. It becomes essential for a physician to start the treatment after determining his temperament so that the patient having specific mizaj can be treated according to his own mizaj. In this paper we have discussed the determinants of temperament as described in literature of Unani system of medicine.