Fundamental concepts of Bragg coherent diffraction imaging （BCDI） are briefly reviewed. BCDI is increasingly popular for imaging crystalline nanoparticles under in-situ conditions. In contrast to the conventional coherent diffraction imaging （CDI） that measures a forward coherent scattering in a small angle region, BCDI is based on coherent Bragg diffraction from a crystalline sample for imaging, enabling us to observe the 3D minute displacement field of the lattice planes as well as the electron density of the particle, which provides new insight into the relationship between physical properties and lattice strain/defects.