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Twin tunnels can be used for many applications. Interaction between two tunnels is an important problem in tunnel engineering that should be studied specially. Numerical investigations are well adapted to field data and numerical methods can be used in design of rock pillar of twin circular tunnels. So far no relationship has been provided to estimate the minimum stable rock pillar. In this paper the interaction between twin circular tunnels has been studied using 2D finite element analysis. To do this, a great number of twin tunnels were modeled in Phase2 software with different conditions of rock mass (RMR value) and depth of tunnel. Models were analyzed and minimum stable rock pillar was determined. This process was repeated for three different ratios of K (ratio of horizontal stress to vertical stress, 0.5, 1 and 1.5). Finally, according to the linear and nonlinear regression methods, the best merit function was fitted to result of numerical analysis. Then, a new approximate formula was proposed to predict ratio of width to height rock pillar according to RMR value and depth of twin circular tunnels with different K values. The formulae are very accurate (coefficient of correlation equals to 0.96) that can be used for predicting ratio of width to height rock pillar in twin circular tunnels.