A pot experiment was carried out at the Experimental Station and in the Laboratory of Vegetable and Floriculture Department, Faculty of Agriculture, Mansoura University during the two successive seasons of 2010 and 2011 to investigate the effective role of ascorbic acid at 50 and 100 ppm on alleviation the harmful effect of irrigation by diluted seawater (10, 20, 30 and 40%) on plant growth and biochemical constituents of Hibiscus (Hibiscus rosa-sinensis L.). The results showed that shoot height, number of branches, number of leaves, leaf area, root length as well as fresh and dry weights of shoots and roots were increased at the low levels of salinity of diluted seawater irrigation (10%), and then decreased gradually with increasing irrigation water salinity from 10 to 40% as compared to non-salinized (control). The shortest plants resulted from the highest irrigation water salinity (40%) in both seasons. Most criteria of plant growth were significantly affected by application of ascorbic acid separately or collectively as promoted all the aforementioned characters in this study. In addition, salinity stress decreased photosynthetic pigments, N, P and K %, while increased proline and total soluble sugars. Moreover, AsA increased photosynthetic pigments, proline, total soluble sugars, N, P and K concentration. It could be noticed that the AsA at both levels could partially alleviate the harmful effect of salinity stress on plant growth and biochemical constituents of Hibiscus plant. From the obtained results, It could be recommended to spray Hibiscus plants irrigatied by diluted seawater with ascorbic at 100 ppm to overcome the destructive effect of salinity.