Using "Yonsei$^{101}$ Analog Computer" the poisoning of the Xenon$^{135}$ in a thermal homogeneous nuclear reactor is analyzed. The simulator is constituted of high gain D.C. operational amplifiers and operational impedances. During the nuclear reactor operation, the Xenon poisoning increases against time until the equilibrium state reaches. After the reactor shut-down, it increases remarkably until the maximum value and then decreases. The simulated curves agree with theoretical values satisfactorily. The accuracy of the analog computer solution is 0.4387 per cent during the nuclear reactor operation and 6.7 per cent after the nuclear reactor shut-down respectively.pectively.