Monoclinic β-Ga2O3 nanosheets hold great potential applications in electronic, optical, and photocatalytic fields. In this study, two-dimensional β-Ga2O3 nanosheets were successfully fabricated through a simple crystalline phase transition from the as-prepared ultrathin γ-Ga2O3 nanosheets. The photocatalytic hydrogen evolution reaction under UV light irradiation was achieved on the two kinds of photocatalysts. However, β-Ga2O3 with a higher crystallinity shows a lower photocatalytic activity in comparison with γ-Ga2O3. The average apparent quantum yield is calculated to be 0.29% for β-Ga2O3 nanosheets and 1.82% for γ-Ga2O3. More efficient separation and transfer rates of photogenerated carriers and larger specific areas were found in γ-Ga2O3. On the basis of the analysis of the structures of γ-Ga2O3 and β-Ga2O3, it is proposed that the disordered or defective structure contributes to the improvement of photocatalytic activity to some extent. Therefore, it is significant to develop the photocatalyst with a stable structure and a certain number of defects at the same time.