The effects of carbon addition on the solidification microstructure and rupture life were investigated in five different carbon level single crystal superalloys. With the increasing of carbon level, the volume fraction of eutectic decreased markedly and the volume fraction of carbide increased. The carbides mainly distributed in interdendrite zone, when the carbon level was high, there were little carbides in the dendrite core. After heat treatment, coarse γ/γ′ eutectics in interdendrte zone mainly were dissolved, a little γ/γ′ eutectic was not dissolved. Morphologies of carbide became much simpler, the size of carbide decreased, the volume fraction of carbide decreased, and the distribution of carbide became much more dispersion, and the type of carbide became much more variety. Grain and chainlike M₂₃C₆ appeared after heat treatment. With the increasing of carbon level, the rupture life of single crystal superalloy increased at first and then decreased, and the rupture life came up to the maximum when the carbon level was 0.045%. SEM observation indicates that the cracks of alloys mainly originate from shrinkage, carbides and eutectics. The change of rupture life was mainly because the un-dissolved eutectic and carbides of alloy which act as the source of cracks. The variation trend of carbide and eutectic was contrary with the increasing of carbon level; therefore, the carbon content should be controlled in the perfect level.