Brain Computer Interfaces (BCIs) provide a direct communication channel between human brain and environment. Synchronous BCIs assume that BCI users are always at control state and definitely produce a control command after a predetermined number of flashes. On the other hand, asynchronous BCIs determine the moments that BCI users focus on the BCI system to make command selection and produce control commands only at these moments. At a P300 based BCI system, dynamic stopping function enables to determine the required number of flashes to make command selections, during the command selections. In this study, a P300 based asynchronous control algorithm combined with dynamic stopping function is employed. The proposed synchronous and asynchronous control algorithms are tested on the P300 based BCI system designed for an environment control application. Performance of synchronous and asynchronous BCIs is compared according to task completion duration. According to the results of the study, compared to the synchronous BCI, asynchronous BCI has a superior performance in terms of task completion duration.