<p>We have investigated the second-order nonlinear susceptibility associated with difference frequency generation in singly electron charged CdSe/ZnS, CdSe/ZnSe, CdSe/MgS &amp; CdSe/MgSe quantum dots via intersublevel transitions in the conduction band. The confined energy levels in the dots are calculated in the effective-mass approximation by solving the three-dimensional Schrodinger equation. The second-order nonlinear susceptibility coefficients in the conduction band DFG processes are then determined using density matrix approach. As the dot radius increases, the DFG susceptibility spectrum shifts towards lower energy side. Also, we observe that, the peak height of the DFG spectrum increases slightly with increasing dot radius. For CdSe/ZnS, CdSe/ZnSe, CdSe/MgS quantum dots the second order nonlinear susceptibility associated with DFG is found to be one order higher (≈10⁻¹⁰ m/V) in contrast to that of the bulk CdSe (≈10⁻¹¹ m/V). We observed that second-order nonlinear difference frequency generation processes depend on the polarization state of incident photon beam, dot size and the surrounding matrix.</p>