Boron (B) is added into single crystal superalloys as a micro-alloying element to strengthen low angle grain boundaries. However, there are limited systematic investigations about the effect of B on the microstructures of single crystal superalloy. We studied the influences of B on phase transition temperature, (γ+γ') eutectic and borides by analyzing the quantitative characterization of the as-cast and heat-treated microstructures of 3 kinds of second generation Ni-based single crystal superalloys DD11 with various additions of B (mass fraction of 0, 0.01% and 0.02%). The results indicate that increasing B addition can significantly decrease the solidus and liquidus temperature and enhance the volume fraction of (γ+γ') eutectic. The borides of M₃B₂ are not observed in the alloy with 0.01% B addition. However, when the B addition is 0.02%, the script-like borides are precipitated in the interdendritic region and the incipient melting point is decreased, and thus the volume fraction of residual eutectic is significantly improved. The script-like borides are rich in Cr, Mo and W, which decreases the solid solution strengthening effect, and thus creep properties of the alloy may be significantly decreased. This study would be helpful for the understanding of B's role in Ni-base single crystal superalloys and the optimization of B content.