Understanding the physical mechanism of structural stability and transition in various polytypes of layered transition metal dichalcogenides under the external stimulus is of crucial importance for their new applications. Here, we investigate the thickness-dependent structural properties of MoS2 under the condition of hydrostatic pressure in terms of bond relaxation and thermodynamics considerations. For both types of MoS2 structures, we find that the transition and metallization are significantly modulated by hydrostatic pressure and the number of layers. We establish a pressure-size phase diagram to address the transition mechanism. Our study not only provides insights into the thickness-dependent structural properties of MoS2, but also shows a theoretical guidance for the design and fabrication of MoS2-based devices.