In this work, we analyze and compare the optical properties of spherical A: CdSe–ZnS–CdSe and B: ZnS-CdSe-ZnS core–shell–shell quantum dots (CSQDs). Under the framework of the effective mass envelope function theory, the nonlinear susceptibilities associated with inter-sub-band transitions in the conduction band are computed by solving the three-dimensional Schrödinger equation for A and B QDs in the presence of impurity. We theoretically investigate the third-order susceptibilities and optical absorption coefficients as a function of core radius while the outer radius of quantum dots was fixed. The numerical calculations show that QD size plays a fundamental role in determining the nonlinear optical properties of QDs. The susceptibilities and the absorption coefficients have pronounced single peaks (resonance) and depend strongly on the geometry of these two quantum dots as well as the effect of the quantum confinement. Our theoretical study shows that susceptibility and absorption coefficients peaks are red-shifted by increasing the core radius, and the magnitude of susceptibility and absorption coefficient increase. The resonant magnitudes Im ( ) of A and B-CSQD are negative and are around (-2.4) and (-2.5), respectively. While, for the core radius of R1 = 40 nm, Real ( ) of A and B-CSQD changes significantly near the resonant frequency from positive value (+1.7, +1.6) to negative one (-1.7, -1.6), respectively. Furthermore, for R1 = 40 nm, the absorption coefficient of A-CSQDs has reached a maximum with the magnitude of situated at approximately 0.02 eV. In contrast, this value is equal to for B-CSQD. Our computational results may open a new window in the development of QDs structures for application in optoelectronic devices. تفاصيل المقالة