The critical water depths of eelgrass bed formation were considered to be related to two phenomena. The leaves and roots of eelgrass in the shallower area of upper critical depth will be cut off and washed away by the wave action; eelgrass in the deeper area of lower critical depth will die by the decline of net photosynthesis. Authors elucidated the relationships between the upper critical depth on sandy substratum and wave conditions (0.2 of critical shields number, 0.6m/s of orbital velocity), and also between the lower critical depth and net photosynthetic rate (monthly net photosynthetic rate=0). Net photosynthetic rate of eelgrass is a useful index for the conservation of coastal environment.