Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of 235U are compared with those calculated from the simple statistical model by MADLAND and ENGLAND. The method of calculation has been extended to the isotopes having more than one isomeric stale. The results may be explained according to the multi-exit-channel model of fission.