Numerical analyses of ideal contoured rectangular plug nozzle of DPR (design pressure ratio)=96 considering actual flight conditions were conducted to evaluate its performance. Drastic decreases in thrust efficiency coefficient were observed at transonic flight condition. The effect of intentionally setting the nozzle DPR low was also investigated, and it was found that using nozzle of DPR=19 in the same flight conditions, the transonic thrust efficiency decrease remarkably reduced compared to the DPR=96 nozzle. Furthermore, only 5% decrease in thrust efficiency occurred at cruising condition even though its nozzle size (exit area) is less than 1/3 of the DPR=96 nozzle. Researches were also made to seek for means to further improve the thrust performance during the transonic flight by modifying the nozzle's cowl geometry configuration. Here, thrust efficiency of the DPR=19 nozzle at transonic flight improved up to 2.6% by providing an extended cowl with a slit near the nozzle throat.