Abstract： Objective To provide the methods and evidences for monitoring and protection of pilot's cerebral function(CF). Methods Forty-five male pilots(average age were 27.5±2.6 years,ranging from 25 to 32 years)served as subjects.Mean blood velocity(Vm)in anterior cerebral artery(ACA),middle cerebral artery(MCA)and posterior cerebral artery(PCA)was tested by transcranial Doppler ultrasongraphy during 5 h mental arithmetic tasks and consecutive performance tasks.Neurobehavioral ability index(NAI)and scores of Stanford sleepiness scale(SSS)were analyzed individually. Results When performing mental arithmetic tasks.Vm in ACA increased at the second hour and decreased after the fourth hour,there were no significant changes in MCA and PCA.When performing consecutive performance tasks,Vm in PCA declined earlier than that in MCA after the fourth hour.Vm in ACA,MCA and PCA did not decrease significantly until the fifth hour.The rising trend of NAI Was found after the second hour,while falling trend after the fourth hour.After the fourth hour,the false responding rate of mentaI arithmetic tasks and SSS scores increased significantly which showed depression of CF. Conclusion The effect of cognitive tasks on Vm was different.5 h mental arithmetic tasks didn't have significant effect on Vm in MCA.Significant decrease of Vm in MCA when performing consecutive performance tasks for 5 h may be a factor inducing presyncope or poor orthostatic tolerance.