Micro or mini heat spreaders are used in the interest of providing higher cooling capability for microtechnologies. Heat spreaders using micro or mini channels are not yet well studied, for this the fundamentals of two-phase heat transfer in microchannels are being studied. Here, a comprehensive experimental two-phase flow study has been carried out on two single round tubes (D = 0.509 and 0.790 mm) and for two different fluids: R-134a and R-245fa. An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. Four principal flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were observed. A new type of flow pattern map for evaporating flow in microchannel has been developed. The first zone corresponds to the isolated bubble regime. It includes both bubbly flow or/and slug flow and is present up to the onset of coalescence. The second zone is the coalescing bubble regime. It is present up to the end of coalescence process. The third zone is the annular zone and is limited by the fourth zone of this diabatic map, the onset of critical heat flux. This flow pattern map can be used for heat transfer model and design of micro evaporator. The vapor velocity or cross sectional void fraction have been measured. For R-134a, the flow can be considered to be homogeneous (or near homogeneous). For R-245fa, more tests exhibit instabilities and surprisingly show vapor velocities below those of homogeneous flow. Frictional two-phase pressure drops have been measured over a wide range of conditions for the two microchannels and two fluids. Three regimes are distinguishable when regarding to the variation of the adiabatic frictional pressure drop with the vapor quality or the two-phase friction factor with the two-phase Reynolds number: a laminar regime for ReTP < 2000, a transition regime for 2000 ≤ ReTP ≤ 8000 and a turbulent regime for ReTP ≥ 8000. The turbulent two-phase flows are best predicted by the Müller-Steinhagen correlation. New accurate CHF data have been measured with the test facility. A new microchannel version of the Katto-Ohno correlation has been developed to predict the CHF in circular, uniformly heated microchannels. Moreover, a new transition curve from annular flow to dryout has been proposed.