Two field experiments were conducted on sweet potato (Ipomoea batatas L.) cv. Beaure Gard at the Horticulture Research Farm of El-Bramoon, El-Dakahlia Governorate, during the two successive summer seasons of 2007 and 2008 to evaluate the effects of different rates of phosphorus (15, 30 and 45 kg P₂O₅/fed) either single and/or in combination with application methods of humic acid (control, foliar spray, transplant treatment and soil application) on plant growth, yield and its components, as well as chemical constituents and storability of tuber roots.  In general, results showed that the increasing of applied phosphorus rate from 15 kg P₂O₅ up to 45 kg P₂O₅/fed significantly increased main stem length, canopy dry weight plant leaf area, total chlorophyll and carotenoides as well as total and marketable yield, dry matter percentage of tuber root and tuber root weight and diameter, Moreover, Application phosphorus at 45 kg P₂O₅/fed significantly increased N, P, K, carbohydrate and total sugars in tuber roots. This P-rate had the most interesting observation was the enhancing of storability and reduced decay percentage. On the other hand, application methods of humic acid had a significant effect on all studied characters in both seasons. Soil application method of humic acid had a significant increases in plant growth characters, photosynthetic pigments, total and marketable yield and tuber root quality. Besides, this application method significantly increased chemical composition of tuber roots and reduced the weight loss and decay percentages. The combined treatments of P-rates and application method of humic acid were generally more effective on the most studied parameters than with single ones. The best results were obtained by application 30 kg P₂O₅/fed with soil application method of humic acid. This treatment achieved increases in vegetative growth characters, total and marketable yield, average of tuber root weight and diameter as well as concentrations of N, P, K, carbohydrate and total sugars in tuber roots. In addition, this combined treatment enhanced the tuberous roots storability and reduced decay% comparing with the other ones.  Therefore, this treatment could be recommended for raising sweet potato yield and improving tuberous roots quality as well as reduced the need for chemical P-fertilizer by about 33.3 %, thereby reducing costs and environment pollution under similar conditions to this work.