The isotope ratio of $^{134}Cs/^{137}Cs$ in a spent PWR fuel sample was obtained with a newly developed gamma/neutron combined measuring system at KAERI. Burnup and Pu/U ratio of the spent fuel sample were determined by using the measured isotope ratio and the burnup-isotope ratio correlation equations calculated from the ORIGEN-ARP computer code. The results were compared and evaluated with the chemically determined burnup and Pu/U ratio. As a result of the comparative evaluation, the nondestructively determined burnup and Pu/U ratio values showed a good agreement with the chemically obtained results to within a 4.5% and 0.8% difference, respectively.