A L-serine producing facuhative methylotroph mutant (Mth^H、Gly^R)was derived from Pseudomonas flava A3 by combination treatment with ultraviolet(UV)and diethylsulfate(DES).It could accumulate 6.2g/L L-serine in the medium containing 30g/L glycine and 1% methanol as carbon source when it was cultured for 3 days,and it has an improvement about 67.6% compared with the origin.The mutant also has a good stability of descendiblity of L-serine producing.