In　Chinese　highway　system,　especially　within　mountainous　areas,　there　arose　plenty　of　excessive　long　down-hill　highway　segments.　Accident　documents　from　many　provinces　revealed　many　of　those　long　down-hill　segments　burdened　substantially　with　high　traffic　crash　frequency.　Preliminary　analysis　showed　that　it　was　truck　to　dominate　the　majority　of　crash　involved　vehicles　in　these　down-hill　segments　due　to　malfunction　of　braking　system.　This　paper　concentrates　on　the　traffic　safety　performance　of　down-grade　segments　of　mountainous　highways　to　seek　how　the　traffic　and　design　factors　aggregate　the　risks　of　vehicle,　and　most　importantly,　to　glean　how　the　primary　index,　length　of　grade,　influence　the　safety　performance　of　road　segments.　Specially　highlighted　among　this　paper＇s　findings　is　the　causation　model　between　length　of　grade　and　crash　counts　under　the　interaction　with　other　confounding　factors　such　as　grade　and　curvature　of　horizontal　alignments.　This　outcome　would　hopefully　be　employed　as　the　meaningful　reference　to　highway　geometry　design,　especially　for　the　restriction　of　plausible　length　of　down-hill　highway　grades.