The synthesis of boron nitride nanodisks (BNNDs) with reducing the size and having fewer disk layers, and low optical band gap (E g) is essential for practical applications in electronics and optoelectronic devices. So far, the large-scale preparation of hydroxyl (-OH) and hydroperoxyl (-OOH) functionalized boron nitride nanosheets and BNNDs with reduced E g is still a challenge. This research demonstrates the scalable and solution process synthesis of hydroxyl (-OH) and hydroperoxyl (-OOH) functionalization of BNNDs at the edges and basal planes from pristine hexagonal boron nitride (h-BN) by the combination of modified Hummer's method and Fenton's chemistry. Modified Hummer's method induces exfoliation and cutting of the h-BN into BNNDs with a low percentage of -OH functionalization (6.90%), which is further exfoliated and cut by Fenton's reagent with improved -OH and -OOH functionalization (ca. 17.25%). The combination of these two methods allows us to reduce the size of the OH/OOH-BNNDs to ca. 200 nm with the number of disk layers in the range from ca. 6-11. Concurrently, the E g of h-BN was decreased from ca. 5.10 to ca. 3.58 eV for OH/OOH-BNNDs, which enables the possible application of OH/OOH-BNNDs in semiconductor electronics. The high percentage of -OH and -OOH functionalizations in the OH/OOH-BNNDs enablesg them to disperse in various solvents with high long-term stability.