Even if bulk silicon is poor emitter of light, due to quantum size effect which leads tornan increase (the blue shift) of the band gap of the crystallites compared to bulk silicon,rnsilicon nanostructure (porous Silicon, quantum dots, quantum wells and nanoclustersrn) exhibit strong photoluminescence at room temperature. Nanosilicon research isrngaining tremendous attention in recent years towards the possibility of photonic application.rnIn order to explain optical properties of nanoporous semiconductor i.e the variationrnof absorption coefficient, dielectric constant and oscillator strength with size ofrnnanoporous semiconductor information about the density of states which depends onrnthe dimensionality of the system is very important. To do so, we obtain the density ofrnstates for 3-D, 2-D and 1-D and the dipole matrix element with the help of Kane approximationrnof momentum matrix element of direct band gap semiconductors. Usingrnthe dipole matrix element together with the joint density of states between valencernand conduction band states we calculate the optical absorption coefficient, imaginaryrnpart of the dielectric function as well as the oscillator strength near the band edge forrnnanoporous semiconductor thus we find that the optical absorption coefficient andrnthe imaginary part of the dielectric function are explicit function of photon energyrnand band gap energy but implicit function of the nanoporous semiconductors sizernthrough band gap energy. Our results for these optical parameters are in conformityrnwith other observations