Abstract: This paper illustrates the tendency of real-time application software from pollingloop system,foreground/background system to multitasking system and also the characteristics of multitasking system is presented.This paper analyses typical performances:taskswitch timing,intermpt latency timing,which are procvided by real-time operating system.The relations between performances of real-time operating system and worst case design in multitasking applications are also studied.Finally the performances,which real-time operating system must offer for appication worst case design are put forward.