One of the important parameters in undercut slopes design is the determination of the maximum stable undercut span. The maximum stable undercut span is a function of slope geometry, the strength parameters of the slope material, condition of discontinuities, underground water condition, etc. However, the desired production capacity and therefore the size of excavating equipment will sometimes ask for a wider undercut span. The influence of arching phenomenon in geo-material on the stability of undercut slopes is investigated earlier. It is believed that due to arching effect, some load transfer from the undercut area into stationary remaining side toes leads to a more stable slope. However, the transferred load may result in ploughing failure of side toes. One technique for preventing the ploughing failure is the use of counterweight balance on side toes. In this study, the influence of counterweight size on the stability of the undercut slopes was investigated through a series of numerical model tests using FLAC3D software. It was concluded that there is a meaningful relationship between the counterweight balance size and the maximum stable undercut span where increasing a counterweight size results in a wider stable span. Finally, the numerical results were compared with pre-conducted physical modeling test and a nonlinear relationship was proposed between the counterweight size and the maximum stable undercut span.