The onset of plasticity in cubic boron nitride (cBN) was studied by nanoindentation with continuous stiffness measurement (CSM) option. CSM option allows us to observe elastic-plastic transition in the contact and to measure the yield strength of cBN at nanoscale. In single crystals with low dislocations density sharp elastic-plastic transition (pop-in) was observed as a result of homogeneous dislocations nucleation at shear stress close to theoretical shear strength (G/2π). For near nanocrystalline cBN sample the smooth elastic-plastic transition was observed as a result of propagation of already existing dislocations in the region of contact.