We develop and apply liquid cell transmission electron microscopy (TEM) for real time imaging of nanoparticle interaction and self-assembly. With PbSe nanoparticles as a model system, we study nanoparticle superlattice formation and structural transformation of the superlattice upon ligand exchange. The ligand mediated nanoparticle rotation, nanoparticle two dimensional movement, etc. dynamic information has been obtained by in situ observation. We apply automated image analysis method to estimate nanoparticle interaction forces as a function of inter-nanoparticle distance. Our in situ study provides critical insights on nanoparticle self assembly mediated by ligand interactions.