The paper considers a novel, modified equation for evaluation of relationship between tablet tensile strength, bonding area and bonding strength with inclusion of fragmentation as particle deformation mechanism. Four types of lactose particles for direct compression were assessed for their micromeritic and mechanical properties (compressibility and compactibility), with particular focus on fragmentation behaviour, bonding area and bonding strength. Compressibility properties were assessed using three established models. Walker and Kuentz-Leuenberger models distinguished lactose plastic properties more effectively in contrast to the Heckel model. Spherical agglomerates of lactose were most prone to fragmentation as determined with the fragmentation propensity coefficient and the number of interparticulate bonds. Fragmentation, together with plastic deformation were found to be the governing factors for tablet tensile strength in α-lactose samples, while high bonding force primarily controlled the tablet tensile strength of anhydrous lactose. Tensile strength of all lactose tablets showed best correlation to the ratio of fragmentation propensity and Walker compressibility coefficient, which is proposed as better deformation index, intended to describe the overall deformation properties of lactose more precisely. A novel expression for determining bonding area is proposed, established on the enhanced deformation index, which includes both plastic deformation and fragmentation as bond formation mechanisms.