This work reports a facile method for preparing the new photoactive In(2)O(3) films as well as their implementation in photoelectrochemical (PEC) application. We firstly investigated the relationship between oxygen vacancies (V(O)) and PEC performance and revealed a rule between them. We found that the optimized In(2)O(3-n) sample yielded a photocurrent density up to 3.83 mA/cm(2) in 1 M Na(2)SO(4) solution under the solar illumination. It also gave efficiency as high as 75% over 400 nm in the incident-photon-to-current-conversion efficiency (IPCE) spectrum, which is the best value for an In(2)O(3) photoanode reported. Moreover, the PEC performance of these films is enhanced as the V(O) increased and then decreased with further increasing V(O). This two-side effect means V(O) can favor the photoelectron activation, or act as recombination centers to prohibit the generation of photocurrent. Making highly photoactive In(2)O(3) nanostructures in this work will open up new opportunities in various areas.