Applications with real-time constraints are not only growing in the field of embedded system, but gaining popularity in the desktop environment as well. In order to satisfy the need of users in these aspects, RFRTOS real-time OS based on Redflag Linux is developed. In this paper, the real-time support about real-time scheduling, preempted kernel, and fine-grain timer in RFRTOS is mainly discussed. The experimental results on RFRTOS show that the improvement gives a better result for applications with real time constraint than original Linux kernel.