Effects of side wall geometry on performance of rectangular plug nozzle are studied for pre-cooled turbojet engine. Numerical simulation is used to analyze flowfield structures of plug nozzle with various side wall lengths and edge angles for optimal, over-expanding, and under-expanding conditions. For each, thrust efficiency is obtained and compared with full length side wall nozzle in search of the most costly efficient design configuration. When the sidewalls were contracted to half of their original length, the maximum thrust efficiency decrease was 1.8%, and when the walls were removed, 6-10% decrease occurred. On the other hand, difference in sidewall edge angle barely had effect on the flowfield and thrust performance. Effects of external flow on plug nozzle performance was also studied, and the drag at the boat tail region and the sidewall edges caused 7.2% thrust efficiency decrease at optimal expansion compared to the case without external flow.