Abstract: There is an increasing evidence for the role of cysteine proteases cathepsin B, L and S in cancer progression. Primary structure of human cathepsin S was aligned with other human cathepsins. According to the alignment, the amino acid sequences of cathepsin S shows 43.3, 22, 29, 42, 56.6, 37.7, 43 and 33% identity with papain and human cathepsin W, B, H, L, O, K and C. Amino acid sequence of human cathepsin L shares the highest degree of identity with cathepsin S, whereas low sequence identity was observed in case of cathepsin W. The 3D model of cathepsin S has been constructed using the crystal co-ordinates of human omega protease. The structure of cathepsin S shows that the molecule is folded into two distinct domain designated as left hand, that mainly consists of beta pleated sheet, and right hand, that consist of alpha helix interacting with each other through the extended polar interface. The active site of protease was predicted and mode of binding with substrate was discussed in the light of constructed model.