The effects of ruthenium (Ru) additions on the microstructures of Ni-based single crystal superalloys in as-cast, post-heat treated and long term aging (at 1100 °C) are investigated by using optical microscope (OP), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that Ru additions promote the increasing of eutectic and primary dendritic arm spacing. Because Ru additions lead more refractory elements segregating (e.g. Re and W et al.) to the dendrite arm, the solution heat treatment of the 3Ru alloy is more difficult. The stacking fault energy of the g matrix is obviously decreased after Ru additions. The extend dislocations in the g matrix may impede the movement of the following dislocations during plastic deformation and improve the mechanical properties significantly. During long term aging at 1100 °C, Ru additions has no significant effects on the rafting process, and the type of topologically close-packed (TCP) phase is not changed after Ru additions (mainly P phase). However, the precipitation process of TCP phase is suppressed after Ru additions. As the aging time prolong, the suppression effects is weakening (over 500 h).