From October 2004 until October 2005 water logging experiments on two mangrove species Avicennia marina (Forsk.) Vierh. and Aegiceras corniculatum (L.) Blanco, were carried out on the tidal flat of Yingluo Bay (109°43′E，21°28′N) in Guangxi, China. Three replicate artificial platforms were set up for seedling cultivation. Eight sea level altitude (SLA) treatments on each platform were created, each 10 cm above the neighboring treatment. The surface planes of the seedling substrates equaled the SLAs and ranged from 320 to 390 cm. 359 cm was the altitude of the local mean sea level. After one year of cultivation under the SLA treatments the growth characteristics and some physiological parameters of the seedlings were measured. The results showed that the lower SLA treatments significantly promoted longitudinal development of stems of both species, while the mean height of A. marina seedlings was 6.5 times higher than that of A. corniculatum seedlings. Compared with A. marina seedlings, A. corniculatum seedlings had a higher conservation rate in the existing leaves. The order of biomass allocation for A. marina seedlings was stem> root> leaf, and mean percentages of the three organs to the total biomass were 56.7%, 25.7% and 17.6% respectively. But the order of biomass allocation for A. corniculatum seedlings was leaf > root> stem, and the mean percentages of the three organs were 49.6%, 33.0% and 17.4% respectively. A strong promoting effect of the lower SLAs could be seen in the total biomass of A. marina seedlings. However, the moderate SLA treatments were the favorite conditions for higher biomass in A. corniculatum seedlings. At the same SLA the chlorophyll content of A. corniculatum seedlings was higher than that of A. marina seedlings, but there was no significant difference between the ratios of Chl-a to Chl-b for the two species. In both the leaves and the roots the activities of enzymes processing active oxygen in A. corniculatum seedlings were higher than those in A. marina seedlings, indicating a more stressful condition for the former than the latter. Our experimental results and field observations indicate that the tolerance of A. marina to seawater immersion was greater than that of A. corniculatum. If only physiological characteristics were considered the most favorable SLA for forestation in Beibu Gulf region should not be 30cm lower than the local mean sea level for A. marina, and roughly equal to local mean sea level for A. corniculatum.