In this study, the effect of enzyme inactivation by microwaves or steam on the quality of oatmeal was investigated, and the effect of microwave time on the nutritional quality, digestion characteristics, and pasting properties of oatmeal with complete enzyme inactivation was explored. The results showed that oatmeal treated with microwave inactivation for 40 s showed no significant differences (p>0.05) in the content of β-glucan, crude protein, and crude fat, as well as in the initial gelatinization temperature as compared with oatmeal treated with normal pressure steaming for 30 min. The content of total phenols, total starch, and rapidly digestible starch significantly increased by 43 mg/100 g, 1.66%, and 9.17%, respectively (p<0.05); peak viscosity significantly decreased by 14 cm·g (p<0.05), and the setback value significantly increased by 11.9 cm·g (p<0.05). With increasing time of microwave treatment, the content of β-glucan and total phenols as well as in vitro protein digestibility decreased; however, the contents of crude fat and rapidly digestible starch increased. The content of total phenols and rapidly digestible starch showed significant changes, and the peak viscosity, final viscosity, breakdown value, and setback value all significantly increased. Therefore, the microwave time for enzymatic inactivation had obvious effects on the quality of oatmeal.