Rotating disk electrode systems are widely used to study the kinetics of electrocatalytic reactions that may suffer from insufficient mass transfer of the reactants. Kinetic current density (jk) at certain overpotential calculated by K-L equation is commonly used as the metrics to evaluate the activity of electrocatalysts. However, it is frequently found that the jl is not correctly identified in the literatures. Instead of jK, the measured current density normalized by diffusion limiting current density (j/jL) has also been frequently under circumstance where its validity is not justified. By taking ORR/HER/HOR as examples, we demonstrate that identify the actualjL for the same reaction under otherwise identical conditions from the experimental data is essential to accurately deduce jk. Our analysis reveals that j/jL is a rough activity metric which can only be used to qualitatively compare the activity trend under conditions that the mass transfer conditions and the roughness factor of the electrode are exactly the same. In addition, if one wants to use j/jL to compare the intrinsic activity, the concentration overpotential should be eliminated.