Magnetic fields (either horizontal or vertical) were found to drastically change the morphology of protein (orthorhombic lysozyme) crystals. A pair of {011} faces evident in crystal grown in the absence of a magnetic field disappeared when crystals were grown in the presence of a 10 T magnetic field. This fact suggests that a magnetic field makes the growth rate of {011} faces faster than that of {101} faces and influences the kinetics of crystal growth. Horizontal and vertical magnetic fields tended to align the c-axis of the crystals along the direction of the field. The combination of magnetic orientation of crystals and buoyancy-driven convection due to gravitational field caused the difference in the growth environments for the {110} and {110} faces, resulting in differences in growth rates of these faces and thus differences in the crystal morphologies. The analysis of the crystal shapes showed that the ratio of the crystal growth rate of the top surface to that of the side surface was 0.2-1. Thus, our study shows quantitatively for the first time that the gravitational field influences the crystal morphology via buoyancy-driven convection.