Four alloys TMS-75 (Base), Base+B, Base+C and TMD-103 (Base+B+C) with varying levels of B, C were directionally solidified (DS), and their weldability was investigated by bead-on-plate electron beam welding. It was found that cracking occurred in the weld-metal, as well as in the heat affected zone (HAZ). Both γ/γ’ and/or γ/carbide eutectic contributed to the weld-metal cracking, and the γ/γ’ eutectic was found to be mainly responsible for cracking in HAZ. However, constitutional liquation of carbides seems to have made some contribution also. The B addition had a detrimental effect on HAZ cracking susceptibility, while C addition improved it appreciably even in the presence of B. In addition, no obvious differences were observed in cracking susceptibility of weld-metal in the four alloys. The cracking modes and mechanisms underlying the effect of B and C addition on weldability could be explained on the basis of microstructures of the four alloys. Four alloys TMS-75 (Base), Base + B, Base + C and TMD-103 (Base + B + C) with varying levels of B, C were directionally solidified (DS), and their weldability was investigated by bead- on -plate electron beam welding. It was found that that occurred in the weld-metal, as well as in the heat affected zone (HAZ). Both γ / γ ’and / or γ / carbide eutectic contributed to the weld-metal cracking, However, the constitutional liquation of carbides seems to have made some contribution also. The B addition had a detrimental effect on HAZ cracking susceptibility, while C addition improved it appreciably even in the presence of B. In addition, no obvious differences were observed in cracking susceptibility of weld-metal in the four alloys. The cracking modes and mechanisms underlying the effect of B and C addition on weldability could be explained on the basis of microstructures of the four alloys.