We investigate the influence of Gallium Nitride (GaN) nanocolumns on the water splitting. The results show that with the GaN nanocolumns diameters decreasing from 250 to 170 nm, the photocurrents for the water splitting increase from 0.12 to 0.29 mA/cm(2). The highest photocurrent is similar to 2.8 times higher than that of the planar GaN. The increase is mainly related to the surface states, which will lead to a band bending and help to separate the photo-generated carriers more effectively. Furthermore, with the diameters decreasing, it will reduce the distance for the photo-generated carriers to the surface, which will suppress the recombination and increase the photocatalytic performance during the transport process. The work provides a cost-effective way for the applications of GaN nanocolumns in highly efficient water splitting.