Heavy-duty　diesel　vehicles　(HDDVs)　are　important　sources　of　urban　nitrogen　oxides　(NOX).　Understanding　real-world　NOX　emissions　from　HDDVs　is　important　because　it　has　been　shown　that　NOX　emissions　could　far　exceed　the　desired　limits　and　impact　human　health.　On-vehicle　sensors　for　real-time　emission　evaluations　have　become　commercially　available,　and　China　has　adopted　an　on-board　sensing　(OBS)　pilot　program　in　Beijing.　The　OBS　system　utilizes　information　from　the　engine　data　networks　which　including　real-time　NOX　concentrations　from　on-road　HDDVs.　To　evaluate　the　accuracy　of　OBS　monitoring,　one　of　the　vehicles　in　the　OBS　pilot　program　was　equipped　with　a　portable　emissions　measurement　system　(PEMS)　and　was　evaluated　over　the　course　of　two　days　during　on-road　comparison　testing.　The　vehicle　was　operated　under　normal　conditions　and　under　conditions　simulating　a　tampered　vehicle　with　no　urea　for　the　selective　catalytic　reduction　(SCR)　system.　A　comparison　between　the　OBS　and　PEMS　exhibited　good　NOX　correlations　on　an　instantaneous　and　moving-average　basis.　Although　large　OBS-to-PEMS　discrepancies　were　found　in　a　small　portion　of　instantaneous　measurements,　the　60-s　moving　averaged　NOX　concentrations　exhibited　good　agreements　across　all　the　test　conditions　(Pearson＇s　R　＞　0.95　and　mean　relative　errors　from　-13%　to　+22%).　During　the　tampered　SCR　simulation,　the　average　vehicle　emissions　increased　from　22　g　kg　fuel(-1)　to　48　g　kg　fuel(-1),　where　both　the　PEMS　and　OBS　quantified　consistent　trends.　The　actual　NOX　emissions,　registered　by　the　OBS　over　fifteen　consecutive　days　of　normal　vehicle　operation,　ranged　from　3.29　g　km(-1)　to　6.65　g　km(-1).　A　comparison　with　the　PEMS　suggests　the　OBS　can　effectively　and　accurately　identify　high-emitting　situations　for　in-use　diesel　vehicles.　Recommendations　are　provided　to　improve　the　implementation　of　OBS　programs　in　future　applications.