We report the decrease in interface trap density (Dit) in Al2O3/InGaAs metal-oxide-semiconductor (MOS) capacitors by using electron cyclotron resonance plasma nitridation of the InGaAs surfaces. The impact of the nitridation process on the MOS interface properties is quantitatively examined. The plasma nitridation process is observed to form a nitrided layer at the InGaAs surface. The nitridation using microwave power (Pmicrowave) of 250 W and nitridation time (tnitridation) of 420 s can form Al2O3/InGaAs MOS interfaces with a minimum Dit value of 2.0 × 1011 cm-2 eV-1. On the other hand, the nitridation process parameters such as Pmicrowave and tnitridation are found to strongly alter Dit (both decrease and increase are observed) and capacitance equivalent thickness (CET). It is found that the nitridation with higher Pmicrowave and shorter tnitridation can reduce Dit with less CET increase. Also, it is observed that as tnitridation increases, Dit decreases first and increases later. It is revealed from XPS analyses that minimum Dit can be determined by the balance between the saturation of nitridation and the progress of oxidation. As a result, it is found that the superior MOS interface formed by the nitridation is attributable to the existence of oxide-less InGaN/InGaAs interfaces.