Abstract: Studies of two-phase flow patterns and their transitions during condensation have gained increasing interest because heat transfer characteristics are strongly dependent on flow patterns.Therefore,it is very important to know flow pattern inside a condenser at certain flow conditions and hence to develop heat transfer correlations consistent with the prevailing heat transfer mechanism in each flow pattern. The condensation heat transfer for R134a in two kinds of in-tube three-dimensional (3-D) microfin tubes with different geometries is investigated experimentally.In the range of experimental conditions,six distinguishable flow patterns (annular flow,semi-annular flow,stratified flow,wavy flow,slug flow and plug flow) are recorded by a video camera.The obtained results show that the flow regime transition criterion between annular flow and stratified-wavy flow decreases from Fr equal to 7 to Fr equal to 2 in the Soliman flow regime map.In stratified-wavy flow regime,the average heat transfer coefficients h of the two kinds of in-tube 3-D microfin tubes increase as mass flux increases and the number of microfins in 3-D microfin tube is not the controlling factor of condensation heat transfer performance.The regressed condensation heat transfer correlation from the experimental data of stratified-wavy flow regime is obtained.The disparity of the experimental data is within the limits of ±22%.Combined with the criteria of flow pattern transitions,the correlations can be used for the design of condenser of 3-D microfin tubes.