Directional solidified Mg-1.5Gd (wt.%) magnesium alloy was carried out to investigate the effects of the growth rate on the microstructures under controlled solidification conditions. A Bridgman-type directional solidification furnace with liquid metal cooling (LMC) technology was used to solidify the specimens, which could provide steady state conditions with a constant temperature gradient (40K/mm) at a wide range of growth rate (10-200μm/s). The microstructures were observed to be cellular structures, and the relationship between cellular spacing (λ) and growth rate (V) was established in the form: λ= 130.2827V-0.2228 by using a linear regression analysis, which was in good agreement with the calculated values by Trivedi model. The thermodynamics solidification path calculations by Scheil model and experimental observations confirm that the solidification microstructure in the alloy consists of primary α(Mg) phase and binary eutectic α(Mg) Mg5Gd phase. Meanwhile, the microsegregation of the alloying element predicted by the Scheil model agrees reasonably with the electron probe microanalysis (EPMA) measurements.