Abstract: Crystallization of calcium oxalate (CaC2O4) was comparatively investigated in five gel systems in the absence and presence of potassium citrate (K3cit).The five gelatinous systems include water, sodium chloride, synthesized urine, diluted healthy urine and diluted pathological urine, respectively. In the absence of K3cit, calcium oxalate monohydrate (COM) is the dominant phase. However, calcium oxalate dihydrate (COD) and trihydrate (COT) crystals appeared simultaneously in the gel systems containing sodium chloride or synthesized urine. COD appeared also in the diluted pathological urine, whereas there was not any COD or COT crystals in the diluted healthy urine system. In the presence of K3cit, the percentage of COT increased markedly in the gel systems containing either water, sodium chloride or synthesized urine. A great deal of COD appeared in the diluted pathological urine system, and a few COD and COT appeared in the diluted healthy urine system. The lower temperature, higher ionic strength and the presence of metal ions in the gel systems lead to increase of the percentage of COT crystals. K3cit has the ability to induce the formation of COD and COT, as well as to diminish the sectional area of COM crystals, thus K3cit can prevent and cure calcium oxalate calculi.