In this paper the crystallizing metastable zone of oxalic acid in aqueous solution is investigated by usingthe laser scattering method

which detects appearance of nucleus on line

at 20 ℃

30 ℃

40 ℃

50 ℃

60 ℃ respectively. At the same time

the effect of the rate of stirring

the rate of cooling and impurities on the width of the crystallizing metastable zone is studied. The data of crystallizing metastable zone is obtained. The equations of the width of the first and second crystallizing metastable zone with cooling rate are b_Ⅰ=22.63×(Δθ_maxⅠ)~1.1; b_Ⅱ=2.66×(Δθ_maxⅡ)~1.55. The study provided the theoretical foundation for crystallizerdesign and operation.