The paper proposes an implementation of the realtime μCLinux based on RTHAL(Real Time Hardware Abstract Layer). RTHAL with hard realtimeness runs between hardware and μCLinux,and only with a few modifications μCLinux can have harder realtimeness with a scheduling delay of about 3μs. The paper further proposes an effective method to calculate the size of the FIFO buffer for transferring realtime data stably between RTHAL and μCLinux. An example of realtime video coding proves the method can provide μCLinux with good realtimeness easily and effectively.