Abstract The present investigation studied the effect of boron trioxide (B~(2)O~(3)) addition into flux on the microstructure and mechanical properties of modified 9Cr-1Mo (P91) steel in submerged arc welding (SAW) process. Boron trioxide powder was mixed with commercial flux in different ratios to change the weld metal chemical composition. Optical emission spectroscopy results showed that weight percentage of boron varies from 0.00 to 0.0068 with the addition of boron trioxide into welding flux. Weld metal microstructure without the addition of boron reveals the presence of tempered martensite and ferrite phase, whereas boron-added microstructures highlighted the dominating nature of tempered martensite. Scanning electron microscope analysis also showed that the dispersed carbides especially M~(23)C~(6)were increased with the addition of boron. Weld metal macrohardness profile reveals that boron has the significant effect on the weld metals. The maximum hardness of 342 HV was found for the 12.5% B~(2)O~(3), whereas weld of without boron shows the hardness of 240.8 HV. Boron addition has a positive effect on the tensile strength of weld joints except for the 12.5% B~(2)O~(3). But the toughness of weld zone decreases with the addition of boron.