In this study, the hydraulic conductivity of conventional sodium geosynthetic clay liner (Na-GCL) to coal combustion product leachate (CCP leachate) was investigated. The hydraulic conductivity of Na-GCL was then evaluated in terms of bentonite mass per unit area (μbA) of Na-GCL. The hydraulic conductivity of GCL with 3.0 kg/m2 μbA was high throughout the test and was finally measured as 3.7 × 10–8 m/s. The bundles of fibers on the GCL preferentially conduit the flow across the GCL, resulting in high hydraulic conductivity. The existence of bundles of fibers was approved by performing a dye test on the GCL. When the μbA of GCL increased to 4.0 kg/m2, the hydraulic conductivity of the GCL decreased to 2.4 × 10–11 m/s, which is several orders of magnitude lower than that of GCL with μbA of 3.0 kg/m2. The hydraulic conductivity stayed low when the μbA was further increased to 5.0 kg/m2. This result showed that the critical μbA for the conventional Na-GCL was 4.0 kg/m2, where the hydraulic conductivity was well below the allowable limit suggested for the barriers (i.e., 10–9 m/s).