In this paper we present the preparation of PbS nanocrystalline thin films using Chemical Bath Deposition (CBD) technique. We have performed this work in order to study the photoconductivity of PbS semi-conductor thin films. The details of the preparation method are described. Thickness of deposited films has been determined using mechanical and optical methods. From the optical absorption measurements we have determined the band gap values. Using the first approximation parabolic bands model and the obtained values of band gaps, we have determined the size of PbS nanocrystallites. Also, we have investigated the electrical and photoelectrical behaviors of the PbS films. Our study shows that the size of PbS thin films nanocrystallites affects the photoconductive properties of the material. Furthermore, investigations show that there are two different sizes of grains located in two different layers, the first one, with grain’s size of about 25nm, concerns the part of PbS deposited directly on the glass substrate and the second layer, with grain’s size of about 70nm, concerns the PbS deposited on the first layer.