Present investigations deal with ramp induced shock wave and boundary layer interaction (R-SWBLI) in the presence of leading edge bluntness. A second order accurate finite-volume compressible flow solver is employed to assess the effectiveness of leading edge bluntness in reducing the separation bubble size. Boundary layer edge Mach number and temperature, sonic height, boundary layer thicknesses, skin friction coefficient and pressure difference are considered for this assessment. Present studies reveal the existence of two critical radii of leading edge bluntness associated with R-SWBLI. Increase in separation bubble size has been observed with increase in leading edge radius until the first critical radius called as 'inversion radius' which corresponds to maximum extent of separation. Swallowing of the entropy layer by the boundary layer is seen to increase the separation zone size while the separation bubble length decreases when the boundary layer is immersed within the entropy layer. The inversion radius is shown to be associated with equal thicknesses of the entropy and boundary layers. Increase in leading edge radius beyond the second critical radius, called as 'equivalent radius', is found to decrease the size of separation zone in comparison with that of the sharp leading edge case. This reduction is attributed mainly to the existence of a wide over pressure region. Physics-driven predictive strategies, to determine the critical radii for given geometry and freestream conditions, are derived successfully from the present numerical simulations. Current studies on a 50. mm long plate with 15° compression ramp for Mach number 6 and wall temperature of 300. K show that the inversion radius lies between 0.3 and 0. 6. mm while the equivalent radius lies between 1 and 1.2. mm. © 2014 Elsevier Ltd.