IRI　(International　Roughness　Index)　evaluation　criteria　are　studied　based　on　passengers　physiology　and　psychology　reaction　in　this　research.　In　the　test　process,　three　kinds　of　data　are　collected　simultaneously.　The　first　kind　of　data　is　passengers　dynamic　ECG　(electrodiagram)　data　which　represents　the　human　body＇s　reaction　varying　with　riding　vibration　acceleration;　the　second　kind　of　data　is　vibration　acceleration　data　which　is　caused　by　the　road　IRI　value　and　driving　speed　as　measured　by　the　three-axis　accelerate　meter;　the　third　kind　of　data　is　pavement　roughness,　which　is　measured　by　the　laser　road　condition　test　vehicle　in　field　test　and　is　calculated　by　a　computer　procedure　in　a　simulation　test.　The　simulation　and　field　tests　all　prove　that　there　exists　a　correlation　between　passengers＇　HRV　(heart　rate　variability)　and　vertical　vibration　weighted　root-mean-square　acceleration　(defined　as　aw).　When　aw　increases,　the　HRV　decreases.　It　was　further　proved　that　passengers　will　feel　more　uncomfortable　when　they　suffer　a　bigger　aw.　On　the　other　hand,　both　test　results　testify　that　there　is　a　linear　corelationship　between　IRI　value　and　aw　at　the　same　driving　speed.　Ultimately,　based　on　the　field　test　IRI∼　aw　curve　and　the　vibration　evaluation　criteria　of　ISO2631　(1997),　an　IRI　evaluation　guideline　is　established.　The　new　IRI　threshold　will　be　helpful　to　objectively　evaluate　asphalt　pavement＇s　riding　comfort　and　assist　in　the　better　maintenance　decision-making.　©　2010　ASCE.