In the present study, a computation study is performed to investigate the effect of imposed oscillation of nozzle pressure ratio (NPR) on the flow structure in a two-dimensional, axisymmetric supersonic converging nozzle. In this study, the underexpanded flow conditions are considered which are dominated by diamond shock-cell structure. The computational results are well validated with the available experimental measurements. The flow is initially computed to be fully developed and then oscillations are imposed. NPR is increased from 1.6 to 2.6 and then decreased again to 1.6 and thus completes a cycle. Results showed that the external flow structure of the nozzle is dependent on the process of change of pressure ratio during the oscillation. Distinct flow structures are observed during increasing and decreasing processes of the change of pressure ratio even when the nozzle is at the same NPR. Irreversible behaviors in the locations of jet centreline axis and off-axis as well as expansion, compression and neutral zones, are observed at the same NPRs during this oscillation. Further, the effect of oscillation frequency is explored on this irreversible behavior at 100 Hz, 200 Hz, 500 Hz and 1000 Hz frequencies.