A study on the effect of buffer control on user-level QoS of media flows is presented. In multimedia systems, playout buffer at the destination site is often adopted to compensate the delay jitter and improve the continuity of information playback. Buffer contol can reduce the effect of the delay jitter but increase the end-to-end delay. As delay and delay jitter are both the user-perceived QoS parameters, how does buffer control affect user-level QoS? By utilizing the former results on QoS mapping from application-level to user-level, and by investigating the relation between the buffer control parameter, end-to-end-level QoS parameters and application-level QoS parameters, the relationship between the buffer control parameter and user-level QoS parameter is obtained. The effect of buffer control on user-level QoS having been studied in-depth with theoretical analysis. The buffer size providing determinate delay and delay jitter guarantee is found and the buffer size providing the optimal user-level QoS for a certain network condition is demonstrated. Experimental results validate the analysis.