Mild alkali environments are a common condition during starch isolation, processing, or modification, accompanied by hydrothermal treatment, but the effect of mild alkali environments on starch properties remains inadequately studied. The objective of this research was to determine the effect of mild alkali treatment on different starch properties. Normal maize starch (NMS), waxy maize starch (WMS), normal rice starch (NRS), and waxy rice starch (WRS) were subjected to mild alkali treatment (pH 8.5, 9.9, 11.3) under two temperature–time combinations (25 °C for 1 h and 50 °C for 18 h). Their morphology, composition, crystalline structure, pasting properties, gelatinization properties, and rheological properties were determined. The results showed that mild alkali treatment induced pores on the surface of starch granules, and obvious damages occurred in waxy maize and rice starch granules. The protein and lipid contents of starch were significantly reduced by mild alkali treatment. Crystalline structure of starch was unchanged by mild alkali treatment, but the gelatinization enthalpy of waxy maize and rice starches increased with the increase in alkali intensity. The pasting viscosities and storage modulus of starch increased first and then decreased with the increase in alkali intensity. With mild alkali treatment, the consistency of normal maize and rice starch pastes increased and the flow index decreased, showing the increase in pseudoplasticity. And their yield stress and area of thixotropy ring also increased. This study showed that the mild alkali treatment reduced the starch granules' stability by removing proteins and lipids, but induced the rearrangement of amylopectin in waxy starch via ionization of hydroxy groups.