Abstract: Objective:To determine the monosaccharide compositions in polysaccharide from Ulva lactuca L polysaccharides.Methods:The polysaccharide was extracted by ultrasonic-assisted technology, and hydrolyzed with trifluoroacetic acid ( TFA) . The monosaccharide compositions were identified by TLC & PMP-HPLC. Results:The result of TLC analysis showed that the monosaccharide compositions were mannose, ribose, rhamnose, glucose, galactose and xylose.The result of HPLC analysis showed that the monosaccharide compositions were mannose, ribose, rhamnose, glucose, galactose and xylose and their molar ratio was about 0.08∶0.08∶1.00∶0.97∶0.04∶0.68. Conclusion:TLC method could be qualitative to monosaccharide composition of polysaccharides, but it was difficult to quantitatively measure.However PMP-HPLC was a simple method with high sensitivity, good resolution and integrity information.This method was simple, sensitive and accurate, and it could be used for determination of polysaccharide compositions in Ulva lactuca L, whether qualitative or quantitative.