Crystallization of calcium oxalate (CaOxa) was investigated in the presence of sodium tartrate (Na2 tart) of different concentration in a gel system by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. It was found that in the presence of a low concentration of Na2tart (0.01 mol x L(-1)), calcium oxalate monohydrate (COM) is the dominant crystal phase. The percentage of calcium oxalate dihydrate (COD) is less than 5%. In the XRD pattern the main diffraction peaks are (101), (020), and (202) planes of COM crystals. In the FTIR spectrum, the main asymmetric carbonyl stretching band nu(as) (COO-) and the metal-carboxylate stretch nu(s) (COO-) of COM occur at 1 618 and 1 317 cm(-1), respectively. With the concentration of Na2 tart increasing to 0. 10 and 0. 50 mol x L(-1), the percentage of COD crystals increases to 10% and 50%, respectively, and the surface area of COM crystals decreases. The diffraction peaks that assigned to the (200), (211), (411), and (213) planes of COD crystals become stronger and stronger. The temperature can influence the modulation of Na2 tart in terms of the phase composition and morphology of CaOxa crystals. As the temperature decreases, the percentage of COD increases. At a higher temperature beyond 47 degrees C, COM is the dominant phase; but COM is dominant when the temperature is lower than 27 degrees C. The nu(as) (COO-) and nu(s), (COO-) of COD crystals occur at 1 647.6 and 1 327.7 cm(-1), respectively.