In　this　paper,　the　concept　of　mobility　reliability　is　proposed　with　an　assumption　that　the　network　has　a　binary　operation　mode　for　uncongested　and　congested　states　which　are　distinguished　by　a　given　level　of　service　(LOS).　And　the　unit　mobility　reliability　is　evaluated　with　statistical　method　based　on　historical　data,　while　mobility　reliability　at　network　level　is　evaluated　based　on　weighted　average　of　the　unit　reliability　over　an　area.　According　to　the　speed　data　for　three　months　derived　from　floating　cars,　the　road　mobility　reliability　at　both　road　unit　and　network　level　in　Beijing　had　been　calculated;　and　then,　the　possible　influencing　factors　that　affect　the　mobility　reliability　were　analyzed.　The　result　shows　that　mobility　reliability　of　urban　road　network　has　a　certain　correlation　with　network　saturation　(V/C　ratio),　road　network　density,　etc.　©　2011　ASCE.