Abstract: An elementary theory on optical absorption enhancement of nanometer crystallites is presented by using the effective mass model and finite depth potential well. In the strong confinement region, the confinement from the boundary imposed on the relative motion of electron - hole pair enhances the optical oscillator strength enormously, the center-of-mass motion of exciton increases the oscillator strength slowly and the change of transition frequency decreases the oscillator strength with decreasing partical size. The overall contribution from the above three terms leads to the enormously enhancement of the absorption coefficient with decreasing particle size. In the weak confinement region, the optical absorption strength per unit volume tends to a constant which is assosciates with the Well-known effed of giant oscillator strength. Comparison between the calculated results based on our model and the model of the inverse cube size dependence in the strong confinement region for nanomter CdTe and CdS has been made and our results are more consistent with the experimental results.