Utilizing synchrotron small-angle X-ray scattering (SAXS) integrated with a microfluidic device, micellization kinetics of a diblock co-polymer, poly(ethylene glycol)-b- poly(caprolactone), was measured in situ with millisecond temporal and micrometer spatial resolution. The evolutionary regimes of polymer micellization, nucleation, fusion, and insertion were directly observed. The five-inlet microfluidic device provided steady continuous mixing of the polymer solution and the antisolvent. The solvent replacement was mainly dominated by lateral diffusion across the hydrodynamically focused central layer, whose thickness could be precisely designed and manipulated from the mass balance of the partitioning streams. Knowing the micellization kinetics of the polymers is essential for the design and optimization of self-assembled polymeric nanostructures. The technique of integrating SAXS with microfluidic devices can be translatable to other systems for a breadth of applications.