Exothermic continuously stirred tank reactor (CSTR) has nonlinear behaviour and often do not respond well to standart linearized system analysis. The purpose of this work is to point out, by examples, some of the difficulties of CSTR which may arise from disturbances in feed temperature and concentration. To overcome these problems nonlinear optimal control has been used. The design of optimal control and its application to the process are in two steps. First obtaining optimal global trajectory. And then optimal local control.