Mechanical interlock is an important component in load transfer capacity in the rockbolting system. It is in turn dominated by the rebar bolt profile configuration. To gain a deeper understanding of their interactions, rebar bolt units under axial loading were studied. Two kinds of failure mode have been identified, namely parallel shear and dilational slip failures. Based on some assumptions made, a universal upper limit of slipping angle of dilation slip failure can be found as the complementary angle of the grout material internal friction angle, which is also the minimum value of the bolt rib face angle. This theory can explain similar performances of rock bolts in pull-out tests while their face angles are large. In addition, once the geometric parameters of a rebar bolt profile are provided, more narrow slip angles range can be figured out via simple plots. As a result, the grout between the bolt profiles can be recognised as three sections, one works as part of the bolt profile, one carries out shear failure and the bottom part keeps intact.