In this study, the new dimensionless values were defined and proposed to determine the parameters of urban runoff models based on the relative sensitivity analysis. Also, the sensitivity characteristics of each parameter were investigate. In order to analyze the parameter sensitivities of each model, total runoff ratio, peak runoff ratio, runoff sensitivity ratio, sensitivity ratio of total runoff, and sensitivity ratio of peak runoff were defined. $$Total;runoff;ratio(Q_{TR});=;frac{Total;runoff;of;corresponding;step}{Maximum;total;runoff}$$$$Peak;runoff;ratio(Q_{PR});=;frac{Peak;runoff;of;corresponding;step}{Maximum;peak;runoff}$$$$Runoff;sensitivity;ratio(Q_{SR});=;frac{Q_{TR}}{Q_{PR}}$$ And for estimation of sensitivity ratios based on the scale of basin area, rainfall distributions and rainfall durations in ILLUDAS & SWMM, the reasonable ranges of parameters were proposed.