The　flow　characteristics　of　solenoid　valve　region　of　electronic　unit　pump　(EUP).　EUP　was　rebuilt　by　using　optical　visualization　observation.　A　diaphanous　glass　window　was　designed　in　the　pump,　and　the　transient　flow　in　the　solenoid　valve　outlet　was　observed　by　a　high-speed　camera.　By　gray　processing　of　pictures,　the　brightness　curve　within　the　control　valve　region　was　acquired.　Results　showed　that　cavitation　occurred　in　control　valve　of　EUP　and　formed　the　two　phase　flow　phenomenon.　The　formation　of　cavitation　had　two　factors,　one　was　due　to　flow　region　in　control　valve　where　high-speed　fuel　flow　within　was　quiet　narrow,　another　reason　was　that　the　control　valve　hit　the　baffle　which　caused　vibration　and　impingement,　hence　the　fuel　was　extruded.