Abstract: Electron spins in quantum dots are good carriers of quantum information. However, they have large dephasing mainly due to the nuclear spins via hyperfine couplings. Meanwhile, this coupling also provides us a way to dynamically polarize the nuclear spins, which will not only make the dephasing time of electrons larger, but also let us to utilize the nuclear spins to store and transfer quantum information. In this ongoing work, we study the dynamics of nuclear spins (e.g., how the polarization built up) in double quantum dots, which are exposed in inhomogeneous magnetic fields produced by a nano-magnet.