Dynamics of the 1st order phase transition between the U2R2 and the high field phases (HFP) of nuclear ordered solid He-3 was studied by field-cycling method between the two phases. Single Crystals of U2D2 He-3 were grown at the bottom of the compressional cell in superfluid He-3-B at about 0.5 mK. The domain distribution in the U2D2 crystal was determined by ULT-MRI (Ultra Low Temperature - Magnetic Resonance Imaging)(1). We have measured evolution of the HFP after the static magnetic field was suddenly changed to B = B-C1 + deltaB through the lower critical field B-C1. The HFP developed exponentially in time and showed two stage evolution with a fast stage rate and a slow stage rate. We observed that the fast stage rate increased with Delta B. Similar behavior was observed in the reversed phase transition. MRI measurements indicate that this phase transition occurs, at least for small Delta B, by the heterogeneous nucleation mechanism, where new seeds appear at the liquid-crystal interface.