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The weldability of four directionally solidified alloys TMS-75 (Base), Base+B, Base+C and TMD-103 (Base+B+C) were investigated by bead-on-plate electron beam welding, and the heat affected zone (HAZ) microstructures obtained during welding were characterized by optical microscopy and analytical electron microscopy. The presence of / eutectic was observed to be mainly responsible for the cracking in the HAZ, although other mechanisms such as the constitutional liquation of carbides also made some contribution to it. The C addition improved the cracking susceptibility in HAZ appreciably, while B addition showed a detrimental effect on it, however the C addition to Base+B alloy, completely attenuated the deleterious effect of B. The cracking modes and the mechanisms of the effects of C and B additions on the weldability could be rationalized on the basis of various microstructural constituents present in the four alloys.