The streamlined shapes of airborne radomes tend to cause severe degradation of the electromagnetic (EM) performance, which could be compensated by properly designing the thickness profile of the radome. Boresight error (BSE) and transmission loss (TL) are the two most important EM characteristics that need to be improved. Previous researches focused on the single-objective optimization of either BSE, or the linear combination of BSE and TL through the conventional weighted aggregation (CWA) method. However, the single-objective optimization is limited in reaching mutual balance between BSE and TL, and could not explain why the optimal solution obtained by optimizing BSE usually has a better TL than the constant thickness radomes. In this communication, the multiobjective particle swarm optimization (MOPSO) combined with 3-D ray-tracing method is employed to optimize the BSE and TL simultaneously for airborne radomes. The simulation results explain why the optimal solution on BSE usually has a better TL; the TL wall phenomenon in the multiobjective space is also explained. Both problems are due to the variation of mean incident angle of the radomes during antenna scanning, which is general for streamlined radomes. Significant conclusions are drawn and could benefit the radome design in engineering applications.