The analysis of the small angle X ray scattering(SAXS) data was based upon particle characteristic function, one dimensional electron density correlation function and particle distribution function. The microstructure of nylon 66 with different degrees of crystallinity was studied by means of X ray scattering method. The radius of gyration R g, the Porod radius R p, the thickness of crystalline region L c, the thickness of non crystalline region L a, the thickness of interphase region d tr , the long period L , the semiaxises of particles( a, a, b ), the distribution of the particle sizes and the scattering invariant were calculated. The results indicate that there was a significant interphase region between the crystalline region and the non crystalline region, and its content( W t,x ) should not be neglected in comparison with that of crystalline region W c,x . The morphology of nylon 66 prepared by isothermal crystallization at a high temperature was mainly a lamellar structure, while the spherical crystals dominated in the quenched sample. The size of the particles in the quenched sample was smaller than that of those in the isothermally crystallized sample, and the distribution of the particle sizes in the isothermally crystallized sample was wider. The analysis of the small angle X ray scattering (SAXS) data was based upon particle characteristic function, one dimensional electron density correlation function and particle distribution function. The microstructure of nylon 66 with different degrees of crystallinity was studied by means of X ray scattering method . The radius of gyration R g, the Porod radius R p, the thickness of crystalline region L c, the thickness of non-crystalline region L a, the thickness of interphase region d tr, the long period L, the semiaxises of particles (a , a, b), the distribution of the particle sizes and the scattering invariant were calculated. The results indicate that there was a significant interphase region between the crystalline region and the non crystalline region, and its content (W t, x) should not be neglected in comparison with that of crystalline region W c, x. The morphology of nylon 66 prepared by isothermal crystallization at a high temperature was mainly a lamellar structure, while the spherical crystals dominated in the quenched sample. The size of the particles in the quenched sample was smaller than that of those in the isothermally crystallized sample, and the distribution of the particle sizes in the isothermally crystallized sample was wider.