This study evaluates the real-world emission impacts associated with the transition from the CHINA V–VI standards for heavy-duty diesel trucks (HDDTs). On-road measurements were obtained from 24 HDDTs using portable emission measurement systems to quantify particle number (PN) and nitrogen oxide (NO_x) emission factors (EFs) under diverse operating conditions. Comparisons were conducted among CHINA V vehicles, CHINA VI-compliant vehicles, and CHINA VI-noncompliant vehicles, alongside an assessment of selective catalytic reduction (SCR) and exhaust gas recirculation (EGR) system performance. Fleet-level results indicate that the average PN and NO_x EFs of CHINA VI HDDTs were 5.48 and 0.75 times those of CHINA V vehicles, primarily due to a substantial share of noncompliant trucks exceeding regulatory limits. Noncompliant HDDTs exhibited increases of 1695.6% in PN and 85.6% in NO_x EFs relative to CHINA V, whereas compliant vehicles achieved reductions of 51.5% and 59.6%, respectively. Operating mode-specific analysis revealed inadequate PN control at low-to-medium speeds and NO_x exceedances under high vehicle-specific power. SCR systems reduced NO_x by 76.6%–96.8%, with performance varying by compliance status and operating mode, while EGR consistently lowered NO_x emissions. These findings highlight the necessity for strengthened in-use compliance monitoring and enforcement under CHINA VI regulations.