The purpose of this paper is to propose a budget-based RTAI (Real-Time Application Interface) implementation for real-time tasks over Linux on x86 architectures, where RTAI provides a light-weight, high-performance interface for hard and soft real-time tasks over Linux. Our revised RTAI API’s are extended to enable programmers to specify a computation budget for each task, and backward compatibility is maintained with the original RTAI design. Different from the past work, we focus on the implementation of budget-based resource reservation for real-time tasks, which is made complicated by the relationship between RTAI and Linux. Modifications of RTAI are limited to a few procedures without any change made to the Linux source code, such as the timer interrupt handler, the RTAI scheduler, or rt_task_wait_period(). The feasibility of the proposed implementation is demonstrated by a system operating under Linux 2.4.0-test10 and RTAI 24.1.2 on PII and PIII platforms.