The field experiments of runoff and erosion process at plot scale using rainfall simulators have been recently more considered, because of both advantages of natural conditions of soil and treatable characteristics of rainfall. A portable rainfall simulator was therefore designed and constructed in Faculty of Natural Resources of Tarbiat Modares University, with a water pomp, three nozzles of BEX: 3/8 S24W and a tripod to get the nozzles to the suitable height of raindrop fall (max 4.5 m). As the full falling cone, the raindrops ejected from the nozzles at the height of 4.5 m could cover the whole area of 1×2 m2-plot at ground level uniformly, due to the pressure of the system. The raindrop’s diameter distribution and velocity were calculated using high speed photography and image processing technique and the uniformity of rainfall on the plot area was then determined using Christiansen coefficient. The results of rainfall characteristics measurements showed that the median diameters of 1.07, 1.00 and 0.98 mm, mean diameters of 1.14, 1.05 and 1.02 mm, mean velocities of 4.53, 4.08 and 4.00 m s-1, kinetic energies of 10.26, 8.32 and 8.00 J m-2 mm-1 and uniformity coefficient of 90, 87 and 91 % were calculated for rainfall intensities of 30, 60 and 90 mm h-1, respectively.