Boron is added into single crystal superalloys as a micro-alloying element to strengthen low angle grain boundaries. However, systematic investigations on the effect of boron on microstructures of single crystal su-peralloys are limitedly reported. The effect of boron on as-cast and heat-treated microstructures was investigated in two experimental Ni-based single crystal superalloys containing 3 wt％ Re. The current results indicated that the volume fraction of (γ + γ′) eutectic and M3B2 borides was evidently increased, while the number of mi-cropores was evidently decreased with the addition of 0.02 wt％ boron. The (γ + γ′) eutectic could not be dis-solved completely due to the lower incipient melting temperature caused by the formation of M3B2 borides. Meanwhile, the M3B2 borides were found to be enriched with indispensable strengthening elements Cr, Mo, W and Re, and this may lower the strengthening effect and cause stress concentration during high temperature creep.