We use scanning tunneling microscopy to investigate the (001) surface of a cleaved SmB6 Kondo insulator. Variable temperature dI/dV spectroscopy up to 60 K reveals a gaplike density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5d band and localized Sm 4f band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin with the presence of spin-orbital coupling and mixed valence fluctuations. These results shed new light on the electronic structure evolution and transport anomaly in SmB6.