Our previous report has shown that the shapes of calcium oxalate crystals formed in a supersaturated solution with glycosaminoglycan varied with glycosaminoglycan species. The present study was conducted to demonstrate the effect of magnesium on the shapes of calcium oxalate crystals with glycosaminoglycan by optical and scanning electron microscopy. In the presence of magnesium, the phenomenon of the plate- or sheet-like crystals parallel growing on the surface of other crystals was observed with each glycosaminoglycan and was enhanced with hyaluronic acid, chondroitin sulfate, heparan sulfate or heparin, more marked in this order. On the other hand, with heparan sulfate or heparin the stratification of the plate-like crystals was observed, whereas with hyaluronic acid or chondroitin sulfate it was not observed. Moreover, the stratification of the crystals with heparin was stimulated more markedly with increasing magnesium concentrations, and the lamella-shaped crystals at low heparin concentrations in the presence of magnesium was similar to the crystals at high heparin concentrations. These results suggest that magnesium stimulates the effect of glycosaminoglycan on calcium oxalate crystal shapes.