The　slug　length　and　the　trailing　Taylor　bubble　velocity　in　an　upward　vertical　slug　flow　were　measured　by　using　the　optical　probes　and　the　EKTAPRO　1000　high　speed　motion　analyzer.　The　correlation　between　the　trailing　bubble　velocity　and　the　length　of　liquid　slug　ahead　of　that　bubble　is　derived　from　the　experimental　data.　Based　on　this　correlation　as　well　as　the　bubble　overtaking　mechanism,　a　model　for　the　slug　length　distribution　at　any　designated　locations　along　the　pipe　is　proposed.　The　predicted　results　are　in　agreement　with　the　experimental　data.