Ordered ensembles, also known as superlattices, have been formed with a large variety of nanocrystals. The structure of nanocrystal superlattices have been extensively studied and well documented; however, their assembly process is poorly understood. In this work, we demonstrate an in situ space-/time-resolved small angle X-ray scattering (SAXS) measurement and use it to probe the assembly of silver (Ag) nanocrystal superlattices driven by electric fields. By measuring the volume fraction of nanocrystal solution as a function of position and time, we show that the electric field causes Ag nanocrystals to migrate to the anode, leading to a rapid accumulation of nanocrystals that assemble into highly ordered superlattices in several minutes. We have quantitively determined the nanocrystal velocity and flux under various field strengths and found out that a stronger electric field drives nanocrystal to migrate at a faster velocity and higher flux, which in turn cause superlattice to nucleate at a higher density and grow at a faster rate. The quantitative information obtained in this study allows a better understanding of nanocrystal self-assembly and can be used to correlate theoretical models with experimental results.