The experimental realization of a Heisenberg-Kitaev model on the honeycomb lattice has been a major topic in a condensed matter physics, because the model predict a novel quantum spin liquid ground state and unusual magnetic phases induced by spin-orbit coupling and electronic correlation. The 5d iridates A2IrO3 was regarded as a prime example but it turned out that a Heisenberg interaction is dominant over a Kitaev one. The newly discovered alpha-RuCl3 has strong indications that it lies in proximate to a Kitaev spin liquid phase. Two successive transitions at 8 and 14 K occur with small ordered moments but little is understood about them. Moreover, elementary excitation of Kitaev spin liquid which can be identified in scattering experiment is still open question. Using inelastic neutron scattering technique, we will clarify novel magnetism related to the Heisenberg-Kitaev model.