In this paper, effect of an introduced cubic defect on electrical and optical properties of cubic quantum dot is studied. Self-consistent solution of the Schrodinger-Poisson equations is used for evaluation of the proposed complex quantum dot structure. Optical properties (quadratic electro-optic effect and the resonant third-order nonlinear susceptibility associated with intersublevel transitions) of the introduced structure are also investigated using density matrix method. The effects of size, height and position (on or off-center) of introduced defect on energy levels, carrier density, matrix element and optical non-linearity of centered defect quantum box (CDQB) are examined. It is shown that increasing the defect size and height, lead to a considerable enhancement in magnitude of the dipole transition matrix element, third-order susceptibility of THG and QEOE and also red shift in resonance frequency. We show that the CDQB has higher nonlinearity (at least 14 times) and tunable susceptibilities, due to increase of the matrix element and modified energy sublevels, compared with quantum box structure without defect. Also, it is shown that the enhancement of optical nonlinearity is approximately independent of defect position which is so excellent from practical implementation point of view. (C) 2008 Elsevier Ltd. All rights reserved.