A diagram to systematically represent the separation performance for a wide range of operating conditions of high performance liquid chromatography is proposed, It is called a separation performance diagram, showing the relationship between the dimensionless parameter related mainly to the material, k(p)’, and the dimensionless parameter on the elution time, uT/epsilon L-1, for the specified values of yield. The parameter, k(p)’, is the capacity factor measured from the center of a rectangle input, It is equal to the value of uT/epsilon L-1 when the yield of the product is 0.5, This parameter becomes k(0)’ for the linear isotherm, The line for the specified yield in the diagram becomes straight when the parameters related to non-linearity, bC(0) and ut(p)/epsilon L, are specified, The purity of the product is determined from these yields, and the operating conditions such as time interval of sample injection t(p) and the cut time T can be obtained from the diagram, Since k(p)’ is equal to uT/epsilon L-1 for yield=0.5, the value of the abscissa k(p)’ is determined from the location at yield=0.5 of the elution curve obtained from experiments, and the diagram can be drawn. The effectiveness of the diagram is confirmed by applying it to obtain the period between successive injections and the cut time on two-step chromatography, It is also shown that this diagram can be applied to the separation of the mixture with interaction among components.