Molecular dynamics simulation was used to simulate the uniaxial tensile deformation of monocrystalline Ni and Ni₅₇Cr₁₉Co₁₉Al₅ alloy models with different cross-sectional sizes in the [100] orientation, the appropriate simulation modle size with stable plastic flow stress was determined. The tensile deformation behavior of monocrystalline Ni and its alloys of the same modle with stable flow stress were further studied. The results show that the monocrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy with smaller modle sizes are likely to form multi-layer twins or deformation twins during the tensile process because of low stacking fault energy. As the cross-sectional side length of modle is greater than 30 times of lattice constant, the flow stress, phase structures and dislocation density in the plastic flow stage tend to be stable fluctuation with the variation of strain. When the monocrystalline Ni and Ni-based alloys with same modle of stable flow stress stage are stretched, the lower the stacking fault energy is, the larger the area of the stacking faults plane during plastic deformation. During the tensile process of monocrystalline Ni and Ni-based alloys, Shockley partials play a leading role in the plastic deformation process. The formation of multi-layer twins is accompanied by dislocation exhaustion, while the formation and annihilation of deformation twins are mainly dominated by the dislocation starvation mechanism.