Sympetrum frequens and Sympetrum infuscatum are known to deposit their eggs on the dry or damp ground in rice paddy fields during non-flooded periods to pass the winter. However, the effects of winter flooding on overwintering eggs of Sympetrum spp. are unknown. We therefore conducted laboratory experiments to determine the effects of the duration of inundation on the hatching of overwintering eggs. We found that the synchronized hatching coefficients of S. frequens and S. infuscatum exposed to flooding on 20-x-2014, 30-x-2014, 10-xi-2014, 20-xi-2014, 30-xi-2014, and 10-xii-2014 were significantly lower than that of the control. There was no significant difference in the hatching rate of S. frequens eggs between the control and any of the inundation treatments. The hatching rate of S. infuscatum eggs in the October 20 treatment was significantly lower than that of the control. The hatching rate of S. frequens at water temperature treatments of 20°C, 25°C, and 30°C were 16 ± 6.9 %, 42 ± 12.2 %, and 21.3 ± 4.2 %, respectively. Our results based on laboratory experiments indicate that modifications in water regime – e.g., delayed midsummer drainage or the construction of depressions with longer water coverage within the rice paddy fields – are necessary to stabilize populations of S. frequens and S. infuscatum in winter flooded rice fields.