We observed cell structure and analyzed total calcium content and distribution in mango fruits treated with oxalic acid (OA) or calcium (Ca) spray before harvest by transmission electron microscopy combined with in situ precipitation of calcium with an improved technique of potassium pyroantimonate.Compared with controls, the pericarp and outer flesh of OA-and Ca-treated fruits showed a regular, tight cell structure with an increase in starch grains and Ca content. Loose, bound Ca was mainly distributed in the cell wall, cell membrane, tonoplast and plastid, and more Ca was deposited in vacuoles. In contrast, the membrane structures of vacuoles in control fruit cells became obscure. Preharvest OA or Ca sprays may maintain cell structure and increase Ca content of the pericarp and outer flesh of mango, which is beneficial for maintaining fruit firmness and improving Ca nutrition.