The survey of effective factors on rainfall-Runoff relations cause to improve MATHEMATICAL MODELs to analysis that phenomena. For this reason, first of all at 80 experimental plots the correlation matrices among runoff coefficient and some other environmental factors conclude from the topography: plot's length and slop percentage, from the soil: clay, silt, sand all percentage, EC, pH, Strength coefficient, gravel percentage and sum of gravel and litter percentage and from the vegetation cover: canopy and organic matter have been surveyed. In the next step at 77 and after 75 plots two rainfall single events with duration of more than 24h and average intensity of 1.5 mm/h selected and MATHEMATICAL MODEL of accumulated infiltration with relation to plot's length based on regression prepared. After, the prepared MODEL tested by a group of plots with equal condition except length and rainfall depth of 38.2 and 34.8mm. The result is that by increasing the plot's length the accumulated infiltration depth added but the intensity of effectiveness of plot's length on accumulated infiltration depth decrease with increasing length of plot and finally after nearly 22m length the variation of the curve of accumulated infiltration depth with plot's length roughly convert to a linear variation with light gradient.