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The effect of boron addition at 0, 0.007 wt.％and 0.010 wt.％on the microstructure and mechanical prop-erties of K4750 nickel-based superalloy was studied. The microstructure of the as-cast and heat-treated alloys was analyzed by SEM, EPMA, SIMS and TEM. Lamellar M5B3-type borides were observed in boron-containing as-cast alloys. After the full heat treatment, boron atoms released from the decomposition of M5B3 borides were segregated at grain boundaries, which inhibited the growth and agglomeration of M23C6 carbides. Therefore, the M23C6 carbides along grain boundaries were granular in boron-containing alloys, while those were continuous in boron-free alloys. The mechanical property analysis indicated that the addition of boron significantly improved the tensile ductility at room temperature and stress rupture properties at 750 ℃/430 MPa of K4750 alloy. The low tensile ductility at room temperature of 0B alloy was attributed to continuous M23C6 carbides leaded to stress concentration, which provided a favor-able location for crack nucleation and propagation. The improvement of the stress rupture properties of boron-containing alloys was the result of the combination of boron segregation increased the cohesion of grain boundaries and granular M23C6 carbides suppressed the link-up and extension of micro-cracks.