For the quantitative identification of rock behavior in shallow tunnels, we recommend using the rock behavior index (RBI) by combining the analytic hierarchy process (AHP) and the fuzzy Delphi method (FDM). AHP can aid engineers in effectively determining complex and un-structured rock behavior utilizing a structured pair-wise comparison matrix. FDM can assist them to overcome the uncertainty in the expert judgment. Rock behavior types are categorized as rock fall, cave-in, and plastic deformation. Seven parameters influencing rock behavior are determined: uniaxial compressive strength (UCS), rock quality designation (RQD), joint surface condition, stress, ground water, earthquake, and tunnel span. They are classified into rock mass intrinsic, rock mass extrinsic, and design parameters. An advantage of this procedure is its ability to obtain each parameter s weighting. We applied the proposed method to the basic design of Seoul Metro Line 9 and quantified the rock behavior into RBI on rock fall, cave-in, and plastic deformation. The study results demonstrate that AHP can give engineers quantitative information on rock behavior.