The fieldbus control system (FCS) has been increasingly used in the process control area. This paper first shows the timing analysis of the fieldbus control loop by considering both communication and control tasks at the same time. As a result, the fieldbus control loop time was formulated, and the stability condition of the loop operation was represented with the loop time in the FCS. Then a modified PID control algorithm that is suitable for fieldbus devices was proposed to overcome varying communication delays. Finally, a fieldbus control loop was modeled and simulated to validate the timing analysis and the control algorithm. The simulation results showed that the control algorithm works well for the common processes having a first-order model with dead time and the stability of the fieldbus control loop can be assured under the unpredictable and varying time delays caused by the fieldbus.