Objective:①To introduce a method of evaluating mental fatigue,which adopts a kind of working memory test;Using this method to evaluate the degree of mental fatigue induced by 63h sleep deprivation.Methods:8 healthy subjects,aged 21 to 26,were selected to receive the test every 6h during three days,and their behavioral data about mental fatigue were collected.Measurement content includes two kinds of task:the letter content paired task and the letter position task,and each kind of task is set in three difficulty degrees:low,moderate and high,in term of the working memory load.Results: With the SD time prolonged,the mental fatigue scores of two kinds of task,including the three difficult levels,all have the same development trend.Less than SD 16h,there are no significant difference between the baseline value and other time point measurement scores in the six subtests;Whereas,beyond SD 20h,the measurement score shows a significant decline(P<0.01);Then,there is a little recovery;After 48h,the scores keep on decreasing;Although there is a little recovery at the SD 63h,its score is still lower than the baseline value(P<0.01).Conclusion: ①This test is a very sensitive method in evaluating mental fatigue;②During the period of 63h SD,the development of the mental fatigue is not a purely passive course,the advanced nerve center of the brain cortex still has important regulation effects.