Dual-fuel　diesel/natural　gas　direct-injection　engine　is　promising　and　highly　attractive　due　to　its　low-carbon　emission　and　high　thermal　efficiency,　and　both　high-pressure　diesel　and　natural　gas　injections　are　critical　for　air-fuel　mixing.　This　study　presents　an　optical　experimental　investigation　on　the　highpressure　dual-fuel　diesel/methane　injection　process　based　on　a　constant-volume　vessel　test　rig.　The　results　show　that　the　diesel　penetration　process　of　the　dual　fuel　injection　experiences　two　stages:　Stage　I,　the　diesel　tip　penetration　Sdiesel,　the　diesel　spray　area　Adiesel,　and　the　diesel　spray　perimeter　Cdiesel　of　the　dual-fuel　injection　are　smaller　than　those　of　the　single　diesel　injection.　Stage　II,　both　the　diesel　and　methane　continue　to　penetrate　forward,　and　Sdiesel,　Adiesel,　and　Cdiesel　of　the　dual-fuel　injection　become　larger　than　those　of　the　single　diesel　injection　do.　The　diesel　injection　pressure　causes　effect　on　the　dual-fuel　spray　penetration.　The　diesel　injection　pressure　directly　causes　linear　influence　on　the　two-stage　dual-fuel　injection　characteristic.　As　the　diesel　injection　pressure　increases,　the　diesel　spray　meets　the　methane　jet　advancer　and　the　cross　point　occurs　linearly　earlier.　Furthermore,　the　dual-fuel　injection　is　asymmetric　and　the　methane　gas　jet　enhances　this　asymmetry　so　that　the　spray　cone　shifts　to　the　side　of　the　methane　gas　jet.