In this paper, an analytical model is developed to describe the penetration of ceramic/metal lightweight armours by small and medium caliber-projectiles, 4 respectively. Both the projectile and back plate materials are assumed to behave as rigid-perfectly plastic with respect t9, .their nominal stress-engineering strain relationships. Two modes are associated with the back plate and the projectile, respectively, during the penetration 'Ptocess; these are erosion and rigid The model identifies three main phases for the penetration of a lightweight armour; these are: (i) ceramic fragmentation, (ii) penetration of fragmented ceramic and (iii) penetration of back plate. Phase (i) consists mainly of one stage, whereas the other two, i.e. phase (ii) and phase (iii), consist mainly of different penetration stages, respectively. The current penetration stage is ascertained according to the relative velocities between the projectile mass and the mass of remaining ceramic and/or the mass of the back plate ahead of projectile. The main equations representing each penetration stage are derived based on momentum conservation principle. The present model is capable of predicting the time-histories of the velocities of the moving masses and the projectile penetration depth through ceramic/metal lightweight armour as well as post-perforation results. Matching predicted results of the present model with the experimental and numerical results of other investigators serves to determine the flow stress of the ceramic material globally. The present results are concerned with design optimization of ceramic/metal lightweight armours capable of defeating small and medium caliber projectiles. The influence of ceramic thickness and strength as well as back plate thickness and strength on the ballistic limit of the armours considered is presented and discussed. Moreover, the effect of ceramic thickness on projectile remaining mass and residual velocity are discussed.