A graphical method for evaluating the Langmuir favorable adsorption is proposed in this work. The conditional Langmuir constant (K-L(N)), a,new parameter related to the Langmuir equilibrium constant (K-L), can be used to predict the shapes of Langmuir favorable adsorption isotherms. On the dimensionless Langmuir isotherm diagram, all the Langmuir favorable adsorptions (0 < R-L < 1) are further divided into three subgroups: favorable adsorption, very favorable adsorption, and highly favorable or pseudoirreversible adsorption, with two critical Langmuir constants (K-L(N1) and K-L(N2)) as the demarcation point. The feasibility of the method is demonstrated by analyzing 14 adsorption systems. The graphical method is effective and intuitional, and can be used for the favorable evaluation of any Langmuir isotherms. This method shows the advantage that the parameter K-L(N) does not depend on the initial adsorbate concentration (c(o)) over R-L method.