A new biodegradable flocculant was prepared from a common and inexpensive protein. Milk casein was methylated in a 0.05 M HCl methyl alcohol solution at room temperature. The methylated milk casein (MeCS) having a methylation degree of 81% was applied to the separation or flocculation of diatomite in seawater (pH8.1±0.1) at room temperature (18 - 23°C). The flocculating ability of MeCS was evaluated by a sedimentation balance method (cumulative measuring method). The diatomite suspension was effectively flocculated by the addition of a small amount of MeCS (0.25 wt% of the diatomite weight). The results of sedimentation analysis showed that the size-frequency curve had a very sharp and high peak thus the diatomite floc formed by MeCS had a rather uniform size. The settling velocity of diatomite floc at the appropriate MeCS dosages (0.25 - 2 wt% to diatomite) was about 3 × 10-3 m s-1.