Co-Ni-based superalloy exhibits excellent high-temperature mechanical properties and peculiar microstructural development such as: good creep resistance with dynamic strain-aging, ultra-grain refinement by conventional rolling-and-annealing process and very wide extension of staking faults (SFs). These behaviors have been attributed to the chemical interaction between SFs and solute atoms, i.e. Suzuki effect. However, Suzuki effect has not been verified either experimentally or theoretically. In this study, a phase-field simulation for evaluating Suzuki effect has been developed and applied to Co-33Ni-20Cr-10Mo (wt%) alloy which exhibits the peculiar behaviors remarkably. The simulation results suggest that Ni-depletion can be very significant, and the deviation in concentration can exceed 4 wt% at 973 K, for instance. However, the change in the stacking fault energy (SFE) was not so significant to account for the formation of wide SFs. It is suggested that the segregation and/or depletion of minor impurity element such as Nb may be more responsible for the strain-aging than those of main constituent atoms. Thus, the developed phase-field model is useful for giving insight into segregation to SFs. This approach may be applicable to many alloys for optimizing material properties related to segregation to interfaces including SF, twin boundary, phase boundaries and grain boundaries.