Characteristics of the reproductive organs and phenology of Thalassia hemprichii using samples collected from the Andaman Sea, West coat of Thailand, are described. Permanent slides of specimens were prepared by the paraffin method. Preparations were observed under a stereo microscope and a light microscope. The occurrences of reproductive organs were recorded monthly for one year. Staminate and carpellate flowers are actinomorphic. They have three tepals and are unscented. Secretory cells were found in tepals, connective parenchyma of the anther and ovary wall. The pollen grains are spherical and 30-50 μm in diameter. Numerous long papillae were found on the stigmas. One to nine viviparous seeds were observed per fruit. The reproductive organs occurred in the greatest number during the lowest tides during the night time and when temperatures were 24 to 26 oC (December to March). The reproductive organs tend to change with water depth. They decrease when the water depth increases. The highest number of staminate flowers (168.67±47.05 m-2), carpellate flower (205.67±41.23 m-2) and fruit (161.17±31.16 m-2) were found at 1.5 m above lowest low water (LLW).