Two field experiments were carried out in Giza and Ismailia govemorates on onion Giza 20 cv. to determine the efficacy of seedling dip and pre harvest spray applications of two calcium forms i.e. calcium nitrate [Ca (No3)2 ) or calcium chelate on rot occurrence after 2 and 4 months of storage. Correlation with calcium content and Ca accumulation in cell wall of healthy onions was studied. In both experiments, significant reduction in storage rots (neck, black and basal rots) occurred due to the application of each of Ca forms. In Ismailia experiment the incidence of storage rots decreased as the Ca rate increased. However, in Giza experiment, only after 4 month-storage the decrease in storage rot incidence occurred with the medium Ca rates (2% of Ca _(NO3)2 0.2% of calcium chelate ) in addition to the absence of neck rot. Upon the application of Ca (No3)2 at 2 % or calcium chelate at 0.2 %, the greatest reduction in either basal and black rots was experienced with the highest Ca content Scanning electron microscope (SEM) observations revealed cell wall structure integrity as a result of application of either calcium forms at the highest rates.