Determination of 135Cs and 137Cs/ 135Cs ratio by neutron activation method is introduced in this study. Cesium isotopes in various radioactive and radwaste samples were chemically purified before neutron irradiation. 135Cs and the corresponding 137Cs/ 135Cs ratio were determined by measuring the characteristic gamma rays emitted from the activated nuclide 136Cs. Experimental results show that the neutron activation is a practical analytical method for determination of 135Cs and is convenient for researchers in a nuclear-based facility. The isotopic ratio can be used to explain the possible origins as well as age of a radioactive or radwaste sample.