The growth rate of the ((1) over bar 20) and (011) faces of an L-alanine crystal from aqueous solution was investigated in situ using an optical microscope system within a static and homogenous magnetic field from 0 to 5T. A clear inhibition in the growth rate of the two faces was observed in a magnetic field. This growth inhibition effect increased with an increasing magnetic field intensity. The growth inhibition induced by the magnetic field was attributed to a lower mass transfer, which was assumed to be caused by a decreased flow velocity of buoyancy convection and crystal orientation in a suspending state in the solution.