Creep tests have been performed at 760degreesC and 840 MPa on the AM1 and MC-NG nickel-based single crystal superalloys suited for gas turbine blade applications. The stress rupture life of MC-NG is slightly longer than that of AMI but the creep behaviours of the two alloys are very different. Clear relationships have been established between the operative deformation mechanisms and the primary creep behaviours. Occurrence of these different deformation mechanisms is discussed by taking into account the effects of various parameters such as the Orowan stress, the gamma/gamma' lattice mismatch, the stacking fault energy and the solid solution strengthening of the gamma matrix.