RTOS in the embedded system is a powerful tool for the design of multi-tasking. However, previous RTOS has large proportion in the MCU with limited memory. So it is difficult to apply RTOS. In this paper, I removed less frequently used features from the traditional RTOS, and designed lightweight RTOS that schedules and manages the resources with minimal code. I used techniques to obtain user memory using sharing stack, and to reduce the overhead at context. Considering ratio of kernel and applications, the RTOS designed in this paper is available on the MCU with more than 4KB of program memory.