The effective root diameter refers to diameters less than 1 mm, which effectively contribute to the stability of soil aggregate and the reduction of soil detachment rates. However, the appropriate range of this diameter for increasing mean weight diameter of aggregates (MWD) is still unclear. This paper presents a quantitative assessment of effects of root diameter on MWD. The values of root diameter of three plant species were classified into two ranges, 0.3 < D ≤ 0.5 mm, and 0.5 < D < 1 mm and soil samples prepared for MWD measurements. The results showed that the increase in MWD as a function of root diameter could be best described by an exponential equation. The effective root diameter with range of 0.5 < D < 1 can be useful in increasing MWD. A modeling approach has shown that nonlinear regression model between MWD and root diameter was accurate (r2 = 0.76) in predicting MWD, when developed with both size ranges of effective root diameter.