Recent advances in lead halide perovskite quantum dots appeal with their potential in various optoelectronic devices such as photovoltaics, photodetectors, light-emitting diodes (LEDs) and lasers. However, lack of information on the intrinsic optical properties of lead halide perovskite quantum dots (QDs) lags the progress in device performances and further development in various applications. In this letter, the complex dielectric function of CH3NH3PbBr3 perovskite cubic colloidal QDs was determined from the UV-Vis absorption by using a modified iterative matrix inversion (IMI) method. The modified IMI method takes into account the dilute solution with cubic inclusions, while the conventional method only considers spherical or elliptical inclusions by Maxwell-Garnett (MG) effective medium theory. In addition, singly subtractive Kramer Kronig (SSKK) relations have also been considered to compensate for possible errors arising from the finite wavelength range of the experimental absorption data.