Abstract: RTT(round-trip time) fairness in TCP(transmission control protocol) congestion control is analyzed by means of flat graphics and modeling.For the bottleneck link,if using active queue management(AQM),the flow ratio of the two competing TCP traffic streams is approximately the inverse of the RTT ratio,then the RTT unfairness exists.Otherwise,if using drop tail queue management,the obvious RTT unfairness will appear only in the case that the data packets do not time out.If the sending of the packets times out,it will be more complex,that is,large TCP flows of RTT will occupy the network bandwidth,and it is difficult to get a general conclusion.Finally,the NS2 simulations indicate the correctness of the model and the analysis method.