Effect of magnetic field on the shape and crystalstructure of nano MnZn ferrite precursor prepared by co-precipitation way was studied by using SEM, XRD, TEM and TG. It was shown that the morphology of nano particles would change from spheric to chain-like with increasing the magnetic field to a certain critical value. The claviform-like nano particles obtained in a magnetic field of 10 T showed the structure information of single crystal. Thermal gravity curve revealed that the activity of nano particles was reduced in a magnetic field of 10 T due to the increase of one dimension size. The mechanism of magnetic field effect on the nano particle nucleation and growth was discussed by crystal growth theory.