This research shows the dependence of number average degree of polymerization (DPn) of mung bean starch (MBS) on starch slurry concentration, acid/starch slurry ratio, time and temperature in acid modification process. Optimizing these factors to earn as low as possible DPn was evaluated by using the Response Surface Methodology (RSM) with reduced Central Composite Face-Centered (reduced CCF) model. Optimal result is DPn of 795.6 (y) with 29.76% starch slurry (x1), 4.9:1 acid/starch slurry ratio (x2), treating in 120 min of time (x3) and 47.2oC of temperature (x4). Pasting viscosity of MBS decreased after the treatment and DE (dextrose equivalent) was not detected.