This work focuses on the membrane ultrafiltration technique to separate humic acids from aqueous solution. An unstirred dead end ultrafiltration of synthetic aqueous humic acids solutions of 50, 150 and 250 mg L-1 was performed with Polyethersulfone membrane. To observe the effects of applied transmembrane pressure (TMP) drops during separation, the experiments were carried out at 207, 276, 345 and 414 kPa with 50 kDa molecular weight cutoff (MWCO) membrane. The performance of this process is expressed in terms of volume reduction factor (VRF). To analyze flux decline and pore blocking behavior, Hermia’s fouling mechanism models, namely, complete, standard, intermediate and cake filtration models have been studied in search of best fittings in flux decline system. After completion of the experiment with 50 mg L-1 feed solution, the concentration of Humic acids in permeate was 2.9 mg L-1, revealing 94% rejection during the membrane ultrafiltration.