Two-phase f-low is widely elicoi ntt'r',:d in s::verai engineering and industrial fa:ilities. l'he purpose of this study is tc investigate ilt,: clieiar:ter',stic i>f slug countercurrent flov'' 1;attern irr a vertical small diameter pipe. An experittirni.iLi irrvcstigatirl:-r in tiris str"rdy has been perlbrmc'cl tolvard a transparent tube of l9 rnm diameter (oui:ii:ie ,.iiame;UL llL mm. length 200 mm), water alrd air as the Iiquid and gas fluid in the vertical pipe fbll,rr,*'r'(J hv ial..iug a picture of florv pattern in the trzrnsparent pipe using the high-speed camera. J'rio riii-ft:r'ential plessrrre sensors were set in transparent tube in order to receive slug countercurrertt flou'r;:ii.e assccinir:d v,'ith the acquisition data as rnedia to record data and then displayed on the screen. T'rre t\\'r-i-pha:;e ilor,, 'was alrvays preceded by the instabilit-v- of flow rate and bubbles appearance. it occiri'l'ed rrlier tirc steady flow data acquisition rate was iurned c.n for recording the differential pressure. l'lre lesult slrorve<J that the flow pattern characteristic of trvo-phase flow(air-water) in the venicai r,,iFi{ calrsc:rl bv ii"le superficial velocit-v- of a