The problem of router buffer size is one of the hotspots in recent router research.Existing research focuses on traffic pattern

network topology and configuration

router architecture and configuration

network dynamic and performance evaluation criterion.Based on the existing study on evaluation criterion and traffic pattern

this paper proposes a new evaluation criterion named Flow-Completion Time Ratio

which does not rely on the network property.Based on this criterion

we perform simulations using self-similar traffic as traffic pattern.Observing SFCTR

AFCTR and FCU

we conclude that too large and too small buffer size will both degrade the network performance.We also provide reasonable method of setting router buffer size.